To contravene means to go against or defy. You might contravene your parents' ban on sweets when your friend offers to share her candy because chocolate tastes too good to resist!
If you contravene something in practice, you act in direct violation of a particular law or rule. Think about the times when someone has told you not to cross a line and you do anyway. You can also contravene in words though, which means you contradict or argue against a statement. Let's say you're debating gun control. If your opponent says that for the safety of all, it should be legal to carry a concealed weapon wherever you go, you might answer that the more concealed weapons there are, the more violence. You are contravening your opponent's argument.