The President has several important powers written in Article II of the Constitution. When Congress passes a bill, the President can sign it into law to make it official, or veto (reject) it. He may send it back to Congress with changes or simply reject it as a whole. The President also enforces laws across the country through federal agencies like the FBI, the DEA (Drug Enforcement Agency),  and police departments. 

As Commander-in-Chief of the military, the president leads the armed forces during wars or emergencies. The president acts as chief diplomat, meeting world leaders and making treaties (with Senate approval). Finally, the President appoints federal judges, including Supreme Court justices (the Senate must confirm his appointees).