For a legisltive act of Congress to become a law, it has to be signed by the President of the United States. A president can prevent an act of Congress from becoming a law in two ways. #1 ; he vetos it, returns the bill unsigned to the Congress , notifies them it is vetoed, and it does not become a law. However when he does this, the Congress can defeat his veto, by passing the law with a 2/3's majority of Congress, It then becomes law, despite the veto.. #2, with a pocket veto, the president does nothing with the bill, puts it in his pocket so to speak, and neither vetoes it nor signs it into law.When this occurs at the end of a session of the Congress, the bill then expires with the term of the congress, and has to be sent through congress all over again, as a new act of Congress,when Congress next convenes. If the political make up of the congress changes, or if events make the bill a moot point,or if other events become more urgent to deal with, the pocketed bill may never see the light of day again. And the president has avoided any unpopularity/backlash that might attach to an acitve veto