Plain English Meaning
It is the knob on the VOR instrument you turn to pick which course line, measured outward from the station, you want to fly. Once you set the course, the needle on the instrument tells you whether you are on it or off to one side.
Full Definition
The omnibearing selector is the rotatable course-selecting knob and azimuth ring on a VOR indicator that lets the pilot choose which radial (magnetic bearing from the VOR station) they want to track. Turning the OBS rotates the compass-card scale (or moves a digital readout) until the desired course appears under the index, at which the () shows whether the is , , or on that selected course.