I think Wulf's example with the nanosecond dots between beats explained it best. Now lets tweak it a bit so it show bass in relation to drums.
K=kick drum, B = bass, quarternote gets the beat
On the beat
K......K......K......K......
B......B......B......B......
Behind the beat
K......K......K......K......
.B......B......B......B......
Note that the basses quarternotes are shifted a fraction of a second AFTER the kick quarternotes. NOT an eighthnote after, nor a sixteenth or 32nd. The bass is playing EVEN quarters at the SAME bpm as the kick drum. Just consistantly a tad later
In front of the beat
.K......K......K......K......
B......B......B......B......
Same thing as with the behind the beat example, Just done slightly before rather than after.
The important thing to remember is that it doesn't mean playing faster or slower, it means playing the same speed and shifting your position slightly in relation to what is considered the established beat.
This is a hellishly difficult skill to master, as it sounds "wrong" to ears trained to think on beat.
Feel free to add corrections,clarifications or downright dismissals if necessary
