| This topic has probably been addressed before on this forum - perhaps you can search for it.
Anyway, when singing or playing an instrument, you don't have to play precisely in the middle of the beat. You can, instead, play on the leading edge of the beat or play on the back end of the beat.
Imagine a drummer playing a typical 4/4 rock beat all by himself.
You could join in on bass and play your notes ever so slightly ahead of his. You'd then be playing on the leading edge of the beat that the drummer has established.
Or you could play your notes ever so slightly behind his. You'd then be playing on the back end of the beat.
The difference between being in the middle or leading/playing behind is quite subtle, but can make a big difference in the feel of the song.
I notice these differences more with drummers than with bassists.
To create a settled, groovy feel (like much R&B) or a heavy feel (Led Zep, AC-DC), the drummer likes to play on the back end of the beat, ever so slightly behind the other instruments and vocals.
To create a sense of forward motion and surging energy, many rock drummers like to play on the leading edge of the beat (Rush, punk groups). They're striking their drums just a bit earlier than the vocalist and other instrumentalists are singing/playing their notes.
I would venture to say that the same feelings are evoked when bassists lead or lie back.
Hope that helps - good luck! | 
