What's a walk-off? In a baseball game, the away team always bats first (called the "top" of the inning) and the home team always bats second (or the "bottom" of the inning). This means that in the last inning of the game, the home team always knows how many runs they need to score to best the away team. A walk-off occurs when the home team scores a run that gives them the lead in the bottom of the final inning of the game. The home team can immediately "walk off" the field because they’ve already won, instead of finishing the inning.
For example, in the recent 2016 AL Wild Card game between the Toronto Blue Jays and Baltimore Orioles the score was tied 2-2, so the game went into extra innings. By the bottom of the 11th inning, both teams were still locked in a stalemate. The Blue Jays just needed to score a single run and they would win the game. With 2 on base and one out, DH Edwin Encarnacion hit a home run, that won them the game 5-2! They didn't have to keep playing the game even though the Blue Jays still had 2 more outs.