A term used by DJ's which mean to match the beats of two songs and mix them together. This can be done on either on vinyl (records) or plastic (CD's) with the correct equipment, which is turntables or CD decks with a pitch control.
The DJ moves the pitch of the cued song up or down and listens to it on their headphones and compare the beat to the song playing, once they had the beats at the same tempo, the DJ cues the headphone song on a beat, and presses play at the end of a bar of music of the song playing over the speakers.
The song can then be faded in smoothly.