"did not understand" = I did not understand in the past "do not understand" = I did not understand now "what you mean" = what you mean now "what you meant" = what you meant in the past [From most common:] I do not understand what you mean. = I do not understand now what you mean now. I did not understand what you meant. = I did not understand in the past what you meant in the past. I do not understand what you meant. = I do not understand now what you meant in the past. I did not understand what you mean. = I did not understand in the past what you mean now.

Thanks all mates

They're all correct, but they mean four different things (see 马先生's great answer). I think the problem is that you think some words or phrases must always be the same tense, when in reality the tense must reflect the context. Tenses aren't arbitrary - they're used to convey extra information.

Second, third and fourth are right.

I agree with Zoubir.