off the top of my head id say that low speed rebound is that it rebounds slowly and high speed it rebounds fast.
Same for compression, slow speed- compresses slowly, high speed- compresses fast.
I might be right or there could be more than that...

DONT FOLLOW THIS- WRONG APPARANTLY

low speed for big hits, if you drop your bike or do a massive jump, you want to not be cattapulted off when you land so you need the rebound on low speed, for braking bumps and tracks high speed to keep the momentum i should think. Same rules apply for DH bikes.

DONT FOLLOW- WRONG APPARANTLY

I got the wrong end of the stick sorry guys.

Ok mate.

What a load of complete Boll*cks

It is referring to shaft speed

You need low speed compression damping to stop your shock wallowing from rider inputs, like weight shifts

You need high speed compression damping to stop your shock blowing through all of its travel when hit hard, like landing jumps, drops etc.

If your bike wallows and bobs too much then you need to increase your low speed rebound. If your bike blows through its travel and bottoms out too easily then you need to increase its high speed compression damping.

I cant be bothered to explain about rebound damping but I am sure that someone else on here will be happy to oblige.

Sorry, just noticed it was for an MX bike, Not being rude but maybe try an MX forum where there are more people with the info you are after.

He doesnt need an MX forum high and low speed damping are always the same thing, and besides most people on the mx forums i frequent don't have a clue. I'd try having pretty much no low speed then messing around with the high speed and see what happens, or start from the standard settings for your weight in the manual.