This page contains material related to timing in terms of playing “ahead”, “on”, or “behind” the beat. This contributes to the feel of the music.
Some say that other musicians prefer drummers who are a bit ahead of the beat. Playing ahead of the beat pushes the tune forward.
In the same way, playing behind the beat drags the song and gives a relaxed, slow motion.
There is no right or wrong in general, only what works better for a particular piece.
Playing behind the beat is also called “in the pocket”.