A Meganob is what happens when a Nob gets enough Teef to get a skilled Mek to build them orky body armor powerful enough to rival the likes of Terminator Armor. This armor, which is called Mega Armour, turns a Nob into a mini-Deff Dred. Though other Orks mock the Meganobz for wearing so much armor, only the dullest would do so within earshot. Small arms fire simply washes over them, and not even anti-tank weaponry is guaranteed to do much worse than simply knock them over. For many, the only chance of surviving an encounter with an angry Meganob is to outrun him. To counteract this weakness, many Meganobz roar into battle aboard armored Battlewagons. This saves time that would be wasted slogging across the battlefield, and helps the Meganobz get stuck straight into combat.
Ork kulture is a seething orgy of violence where the strong constantly assert their position over upstart, ambitious young Orks. In the extremely tough society of the Orks, only the largest, toughest, and most aggressive Orks rise to leadership. This makeshift "nobility" results in a class of Ork known as "Nobz" in their guttural language. Nobz are larger, richer and more aggressive than normal Orks, and never miss an opportunity to remind their fellows of it. These mighty Orks lead by example, fighting where the fighting is thickest and "encouraging" any recalcitrant Orks or Grots with cuffs and blows to get them into the battle. Orks instinctively obey those larger than themselves, provided they are a healthy shade of green. Most Orks would rather die than bow to a non-Greenskin's will (unless if it's Yarrick). A Warlord's or Warboss' decisions are enforced by this ruling caste.
In the case of Meganobz, they are even bigger and tougher than regular old Nobz, being just shy of the Warbosses and Warlords. While the Warboss commands the Boyz, Meganobz are typically there to provide support and "advise" the rampaging Warboss on how best to krump things. Meganobz are some of the highest ranking Orks within a warband, which means that they are the biggest and strongest. Thus if the Warboss dies, it is not uncommon for the next boss to be drawn from their ranks.
Meganobz are at the top of the Ork pecking order and therefore are able to acquire snazzy and shiny wargear such as Kustom Shootas, Twin-Linked Shootas, Power Klaws, and Killsaws. Sometimes if they are rich enough, they may carry Kombi-Weapons such as a Kombi-Rokkit Launcha, Kombi-Skorcha or a Twin-Linked Shoota. The Mega-Armor they wear is also typically equipped with Stikkbomb launchers.
On The Tabletop
Ork Meganobz can have a mob size of 3-10. A minimum size mob with a Boss Meganob and 2 regular Meganobs equipped with Power Klaws and Kustom Shootas will run you 105 points. In lieu of any other weapons a Meganob can be equipped with 2 killsaws, giving him an extra attack but obviously losing out on any shooting ability except for the stikkbomb launcher.
MegaNobz are very tough, and are highly resistant to low-AP weapons. However they are not quite tough enough to fully shrug off anti-tank weaponry, a problem that is compounded by their lack of an invulnerable save and their slow speed. It would be best to mount them on a Battlewagon or Trukk, or better yet use Tellyporta or Da Jump, to ensure that they get to where they need to be.