How can Congress pass a law that the president vetoed

If a President vetoes a bill, Congress may override that veto with 2/3rds vote in each house (Senate and House of Representatives) and then the bill becomes a law anyways.

By threatening a veto, the President can persuade legislators to alter the content of the bill to be more acceptable to the President. Congress can override a veto by passing the act by a two-thirds vote in both the House and the Senate. (Usually an act is passed with a simple majority.)