From WoWWiki

A trash mob (or simply trash) is any monster that is not a boss.

Trash mobs are usually easier to kill than bosses, although there are some famous trash pulls which are actually harder than the boss they precede (the most recent example is the before-last group prior to High Warlord Naj'entus). Players usually don't like trash mobs, since they are viewed as a tedious and time-consuming preamble to boss fights. Nonetheless, trash mobs in instances usually give reputation and have some useful drops, frequently some crafter material which cannot be obtained otherwise.

Blackwing Lair contains only minimal amounts of trash mobs, many players love it for this reason.

Etymology

The term trash mob derives from the term yard trash which was coined in the late '90s MMORPG Everquest. In that game, a low-level dungeon, Unrest, is composed of a house, which contains several bosses, surrounded by front, side, and back yards. In the yards, there are yard trash. Lower level players shout for groups to kill yard trash for experience either before they are high-leveled enough to kill the bosses or when all the bosses are camped.

Random facts

During the 2007 Blizzcon Dungeon & Raid panel, Blizzard came up with the backronym "Tactically Random A.I. Spawn Heuristic" for TRASH.