Hint: It isn't goldfish crackers, a Tom Clancy novel, or clean underwear.
From Smithsonian:
Contrary to popular belief, the Football does not actually contain a big red button for launching a nuclear war. Its primary purpose is to confirm the president's identity, and it allows him to communicate with the National Military Command Center in the Pentagon, which monitors worldwide nuclear threats and can order an instant response. The Football also provides the commander in chief with a simplified menu of nuclear strike options—allowing him to decide, for example, whether to destroy all of America's enemies in one fell swoop or to limit himself to obliterating only Moscow or Pyongyang or Beijing…
According to former Secretary of Defense Robert S. McNamara, the Football acquired its name from an early nuclear war plan code-named "Dropkick." ("Dropkick" needed a "football" in order to be put into effect.) The earliest known photograph of a military aide trailing the president with the telltale black briefcase (a modified version of a standard Zero-Halliburton model) was taken on May 10, 1963, at the Kennedy family compound in Hyannis Port, Massachusetts.
