The power to Veto any law trying to be passed. It helps keep Congress from going wild with laws, by using a power in its self-called Checks and Balances. (Of course, Congress can override a veto, but it takes a super-majority vote, which is not very easy to do)...
Summarized from a U.S. Government Class POV / Wiki entry: It's all part of the "checks and balances" of the US government. No one branch out of the three has too much power, and the other two have their ways to prevent any branch from getting too much power.