A veto is the ability of the president to reject a law. For example, if the Senate and House of Representatives pass a law they need the president to sign it. The president can refuse to sign it which results in a veto. Then the only way the law would be implemented into American society would be by having the Senate and House of Representatives vote on it again. If both houses vote this law into place with a 2/3rd vote, then this would become law without the presidents help. This process is known as an override.
