A CUE point is a point in the song where something begins (beat, vocals, etc)
Play a song and when you are at the place which you want to use set the point by pressing [SET].
Alter the point if it isn't exact at the place you want by pressing [<<] or [>>]
Now you can jump back to that position by pressing [CUE]
You can also use the Cue function for looping.
When playing the track press [SET] at the exact point you want.
Press [CUE] x beats later to loop back directly.
This needs some practice and don't get angry if it doesn't work the first time.