Pre-delay is quite simply the delay between when a track sends a signal to the reverb plugin, and when the reverb plugin starts processing that signal.
You use it to simulate distance in space. Instruments that are physically further away have a longer reaction time, so you use a higher pre-delay for them.
This is how it’s done with orchestrations anyway, where you try to simulate the entire orchestra on stage, so they sound exactly like you would hear them if you were in the audience. For other genres this isn’t usually so strict in the sense that you try to emulate the physical positions of the instruments, but you can still use it to define the spot of a certain track, up front or more in the back.
it’s easiest to just fiddle around with it. Very high pre-delays will sound unnatural, more like an echo which can also be interesting, but not really realistic