Depends on the context. But generally... A: I need your help with my homework! B: Say no more. I will help you! (Say no more = you don't need to explain further, I will help you right now. But normally it's seen in a more dramatic context)

used to tell someone that it is not necessary to explain something further because one understands what that person is trying to say "A word of advice: don't mention the war." "Say no more—I'll stay off the subject!"