Definition
Going all-in means committing all your remaining chips to the pot in a single action. When a player is all-in, they cannot bet or raise further but remain in the hand and are eligible to win the portion of the pot that they contributed to (called the "main pot"). If other players continue to bet beyond the all-in amount, a "side pot" is created.
All-in situations are pivotal moments in both cash games and tournaments. In cash games, the maximum you can win from any opponent is capped at the effective stack size at the start of the hand. In tournaments, going all-in and losing means elimination from the tournament.
The all-in creates maximum risk exposure and is therefore subject to the most rigorous EV analysis. Short-stack push/fold strategies in tournaments are among the most mathematically analyzed areas of poker, with precise push and call ranges calculated by solvers based on stack sizes, blinds, and table position.
Example
In a tournament final table with 12BB (effective), you pick up A♦K♠ in the Cutoff. Facing a raise and a 3-bet, you move all-in. You are "flipping" against the 3-better's range, but your combination of equity and fold equity makes this an easy profitable shove.