A pocket veto occurs when Congress adjourns before the president signs a bill into law. The United States Constitution states the president has ten days to sign or veto legislation. If Congress adjourns within the ten-day period, then the bill does not become law.
Article I Section 7 of the Constitution states that the president can directly veto a bill by sending it back to Congress with his objections. Congress can override the veto and the bill becomes law if both houses pass it by a two-thirds majority. If Congress is in session and the president takes no action within ten days, then the bill becomes law as if he had signed it. If Congress is adjourned and is unable to receive a returned bill, then it won't become law as if the president had directly vetoed it.