Does it mean anything to "commit under oath" to do or not do something in the future?

I don't understand what that means. The oath of a witness is an affirmation that statements made about what the witness experienced will be truthful, but it seems to me that a promise to do something in the future would require a completely different sort of oath, and not keeping that promise wouldn't qualify as lying to Congress. Is there a law against breaking a promise to Congress?