Plain English Meaning
It's the knob you turn on a VOR instrument to pick which course line you want to fly to or from the station. Once you've picked the course, the needle on the instrument tells you whether you're left or right of it.
Full Definition
The omni bearing selector is the rotatable course-selection knob and associated azimuth ring on a VOR indicator. The pilot turns the OBS to choose a specific magnetic course (radial) to or from a VOR station. The selected course appears at the top index of the compass card, and the () then shows the 's to that chosen course.