Mooks, also known as Minions or Grunts, 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. 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.