"If you leave now, help them you could..." - this referes to Luke helping Han, Leia, Chewie etc to escape (at this point Han wasn't in carbonite)
"but you will you would destroy all for which they have fought and suffered." - Bassically Yoda didn't have confidence in Luke to beat Vader - he thought he'd be destroyed or become a Sith, either would be disastrous for the rebellion as Luke was the key to it's victory.
Essentially loosing Luke would be the undoing of the rebellion (evertything that Leia et all had fought and suffered for).
I think this is one of the best lines in the trilogy as it tells you to weigh up your actions - what might feel the right thing to do right now may not be the right thing to do when you consider the bigger picture. Yoda was suggesting Luke sacrifice his friends for the greater cause.
All about stratergy, self control and sacrifice...