Flak Armour is the most common form of Armor employed by the Imperium of Man and especially the Imperial Guard. Cheap and easy to make and manufacture by the average Imperial world, it is composed of multiple layers of ablative material, shock resistant material and ballistic textiles. Fairly similar to modern body armor in regards of rough construction, though the materials are more advanced. It is resistant to shrapnel and stab attacks done by human combatants armed with regular knife while offering pretty good protection against bullets from combustion guns and glancing hits from directed energy weapons comparable in power to a Lasgun.
How Good Is It?
For example, modern Earth ballistic vests, constructed from Aramids like Kevlar, are either light weight vests that provide reasonable protection only against shrapnel and weaker rounds, such as .45 and 9mm, up to .357 and .44 Magnum or poor penetrating (will stop a shotgun slug but cracked and broken ribs at most, bruising like Mike Tyson punched you at the least), or equip heavier armour inserts that provide more coverage, or possibly a little less and withstand more powerful rounds, all the way up to armour piercing sniper rounds (assuming the wearer has some sort of steel or ceramic insert, and that also gives 3-12 shots from modern assault rifles, depending on generation.)
With Flak armour gives a chance to not only survive a couple of impacts from from a light machine gun, or several from autoguns, but to remain in a shape to continue fighting (assuming it hits the chest piece), all while not weighing down the soldier significantly. When dealing with a local rebellion or facing pirates, bandits, hive-gangsters or the rag tag forces of the lost and the damned, a suit of flak armour is an Emperorsend and often means the difference between survival and a gruesome death. After all, it's said that any stubber at and below rifle caliber will (supposedly) not pierce any part of the Flak Armour. So, in theory, Flak Armor makes you much harder to cut down, and fluffwise, it has also been said a laspistol will not defeat the chest armor. There are two thing to note: First A WH40k Autogun fires bigger rounds than a modern earth Battle Rifle, or LMG. They fire 8.25mm bullets, and this, again, is stopped by Flak. This means the average IG issued gear will take sniper shots with the user surviving. It does beg the question of how big LMGs are in WH40k (real life LMGs use bullets only a bit larger than assault rifles or rather the same size as some nation's assault rifle bullets, so 40K LMGs probably use bullets similar in size to those of their normal rifles). The second is that of course lasguns are not ballistic weapon but energy weapons, so the requirement's to "deflect" a lasgun shot are different from that of a conventional weapon. That it can do both feats at once is no small accomplishment. (Though it is noted to only stop glancing hits from a Lasgun - a laspistol is significantly lower power than your average Lasgun) However, it may be that the angle that it is hit at simply gives enough protection to block a shot.
Note that autoguns are to stubbers what Mass Effect guns are to modern small arms (and can be anything from caseless ammo to rail or gauss automatic weapons to rifles using gravity manipulating technology) and stubbers other than handcannons and heavy stubbers are outright incapable of penetrating flak armor at any range and from any angle and this also extends to the flak shirt and flak pants worn by tunnel rat regiments and potentially what is worn under the armor of all Guardsmen. Flak armor and lasguns are almost uniquely used by the Imperial Guard whereas nearly all rebels and cultists they fight use either mesh armor, armor weave, scrap plate that might eat a stub pistol, or nothing and usually wield stubbers and maybe a few autoguns. Since about eighty percent of the Guard only ever fights rebels and cultists, this makes the average Guardsman practically a Space Marine to his normal enemies which is part of the reason why flak armor and lasgun are standard issue instead of carapace or powered armor. There simply is no point in spending the money to issue better weapons and armor when you're practically invulnerable and invincible to your normal enemies already. When there is something like a WAAAGH!!! or Black Crusade or whatever else, just throw more men at it and lots and lots of tanks and artillery. Since the average Guardsman is nearly invincible against the average rebel or cultist, there are many armored and artillery regiments available to throw at space horrors. It all works out pretty well unless you're one of the ground pounders who has to keep pressure on an anti-tank position while being shredded by anti-infantry you also have to keep busy while your own heavy support gets into position without being killed so they can save your ass so the tanks can come and kill everything. But hey, that's what conscripts are for. Literally.
Also, keep in mind that most aliens the Imperium fights are less advanced than the Imperium. Usually, flak armor and lasgun is sufficient even without overwhelming numbers. Basically, ninety percent of the time the Imperial Guard is to the alien threat of the day what the Tau are to the Imperial Guard (but with more tank and less weeaboo) combined with overwhelming numbers. They really only face a challenge when fighting a true Chaos invasion, Tyranids, Necrons, Orks, and whatever flavor of Eldar (or a minor xenos faction advanced enough like the Fra'al or Tau, which is very rare). Ultimately, the average Guardsman is no joke. Whole squads of modern soldiers will be murdered to death by a lasgun toting IGman. There are literal quadrillions of them, and this is a low estimate. If only the cheap power armor STC were found.
Why It Kinda Sucks
That said, when it comes to protecting its user from guns which shoot .75, or (19.05mm) APHE rocket bullets, hyper-sonic monomolecular ninja star assault rifles, bolts of compressed plasma as hot as a star's core designed to penetrate tank armor, and similar, they are of little more use than a common T-shirt. Of course, if they miss, and shrapnel is thrown, the armor will probably save you, which is its purpose anyway.
Flak armour is also fairly cost efficient; it offers some protection in combat while costing less to produce, ship, and maintain than the soldier wearing it. This becomes less true as the armour becomes more complex.
Also, most enemies are going to be suppressed by the large quantity and concentration of heavy weapons used by the Imperial Guard even without considering the platoon support weapons such as heavy stubbers and the squad special weapons. This enables Guardsmen (were they real) to get close to enemy positions and wipe their foes away with close-range lasgun fire. Since lasgun penetration radically increases the closer they get, this would logically be highly effective. So, most Guardsmen need to to be far more concerned with indirect fire than being hit by enemy infantry directly and flak armor protects very well from indirect weapons such as explosives and shrapnel. Remember, there are literally countless Guardsmen but only "billions die every day" in the Imperial Military as a whole. There is a reason for that.
The next grade up is Carapace Armor. Which is actually a very large leap in quality.