Meaning varies. That which is considered lower-class, visceral, puerile, uncultured. Popular culture as opposed to fine arts. It's usually derogatory, but for no good reason: Airplane! is certainly lowbrow rather than highbrow, but it's one of my favorite movies. The perception of comedy in general as lowbrow may be why critics never seem to review funny movies well.

Presumably meant to evoke the image of a Neanderthal's thick brow.