To successfully override a presidential veto, Congress must

Vote 2/3 in support of what the President vetoed.

Process:

A bill passes through Congress, and must have at least 1/2 support to pass to the President, who will Approve to Law or Veto the bill.

Congress receives the bill again, and must vote 2/2 in support to override the veto.

This can happen when the Executive and Legislative branches do not agree on an issue or may be controlled by different parties.