Mooks, also known as Minions or Grunts or Foot Soldiers and by several other terms, is a slightly old-timey expression used to refer to disposable, low-threat, generic hostile NPCs. These are the bottom of the baddie pecking order; the weakest foes, the chump-change that your PCs will plow through with relatively little effort, if any. Think putties from Power Rangers (and their equivalents in later seasons), goombahs from Mario Bros, and common stormtroopers from Star Wars (who infamously never manage to hit the heroes despite supposedly being elite soldiers), and any other common weak enemy in movies, games, and cartoons that are almost never given personalities and are rarely a serious challenge for the good guys. Some mooks are inferred through the use of low health and/or defenses - others are specially designated. It depends a lot on the system you're using.
Dungeons & Dragons 4th Edition uses the Minion creature role to designate enemies as mooks. They have vastly simplified combat stats, including fixed (and usually low) damage output and a single hit point; these are enemy variants meant to be used in large numbers to "pad out" the combat scene and be something for your party to chop and blast through without hesitation. They are strongly based on 4e's basis as a more narrative-focused "Action Fantasy" type game.
13th Age, being a sort of spiritual successor to 4E, uses Mook as a direct term, usually denoting those minor enemies who would be rather insignificant on their own and unlikely to withstand a single blow, but become more effective as groups.
The FFG Warhammer 40,000 Roleplay system also somewhat toyed with the concept of mooks via the horde system present in Deathwatch and Black Crusade (and to a lesser extent in Only War's Formations) as well as in Wrath & Glory. However, these hordes are somewhat treated as an individual entity, but are afforded several resistances. In exchange, however, they become incapable of avoiding any gunfire (with flamers and grenades being particularly effective) and can thus be ground down, though any attackers will be similarly unable to avoid their massed melee attacks.