This is a term used in football defined as an interception returned by the defense for a touchdown. If the quarterback passes the ball and it is caught by a defender, the defensive team becomes the offensive team instantly. If said player is able to run the ball into the end-zone, they are awarded a touchdown. The "pick" portion of this term is a slang term for interception. The "six" portion of the term is representative of the number of points awarded for a touchdown.
The term "pick-six" can be used in any situation in which a pass is intercepted and returned for a touchdown. The term has been a part of football for many decades, especially as football becomes more and more pass oriented. Another result of the increased passing in football is an increase in the number of pick-six's. Professional football players get faster, bigger, and more athletic every year, which leads to players who are better at anticipating passes and returning the football past the opposing team for a defensive touchdown.