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.