The CUE button sits next to PLAY on each deck and controls where the track starts. On a paused track, pressing it sets the cue point at the current position. While a track is playing, pressing it jumps back to that cue point and stops. Holding it plays from the cue point for as long as you hold it, then returns back when you let go. That is the default “CDJ style” behaviour in rekordbox and Serato, and some software lets you change it.
Not the same as the cue button on the mixer
Each mixer channel also carries a button marked with a headphone icon, CUE or PFL. That one decides what you hear in your headphones and has nothing to do with where the track starts. See Cueing for that control, and hot cues for the several extra points you can save inside a single track.
Origin of CUE button
The button exists because of vinyl. A DJ cueing a record would turn it back by hand to find the exact spot in the groove where the music begins, then hold it there until the moment to let go. CDs cannot be pushed back by hand, so that gesture became a button: Denon DJ shipped a dual CD player with a jog wheel and an instant cue function in 1993, and Pioneer’s first CDJ, the CDJ-500, followed in 1994 with a live cue function.