means A funk-rooted street dance style built on suddenly contracting and releasing your muscles to create sharp, jolting hits in time with the beat.

from From the verb "pop," which has imitated sudden bursts and bangs since Middle English (think of the sound a small explosion or bubble makes). The dance picked up the name on the West Coast funk scene of the 1970s, reportedly because the snapping, jerking motion of flexing and releasing muscles looked and felt like a series of little pops — dancers would even say "pop" on the beat as they did it.