They mean the same thing, but one is the dictionary (plain) form and the other is the polite form. The dictionary form is わかる. In the negative this is わからない。 The polite form of わかる is わかります, and the negative is わかりません。

i think first one means "i don't understand", and the second one means "i didn't understand".

They're informal and formal in that order. Keep on studying, man.

there is no different in meaning,but the second (ます form) is more formal than the first (ない form).