Answer:
A walk-off home run is a home run that ends the game. It must be the run that gives the home team the lead in the final inning of the game.
Addition:
It is named such because after it is hit the defending team literally walks off the field as the game is over. This is a recent term in baseball, however, has caught on well with people around the game and fans. Maybe other then a marvelous defensive play it is the most dramatic way to end a baseball game