AI overview
A king is the most important piece in chess, and the goal of the game is to checkmate your opponent's king by putting it under attack with no legal moves to escape. The king can move one square in any direction and is worth protecting above all else, since losing it means losing the game.
AI-generated from the Wikipedia summary — may contain errors.
Article
The king (♔, ♚) is the most important piece in the game of chess. It may move to any adjacent square that is not controlled by an enemy piece; it may also perform, in tandem with the rook, a move called castling. If a player's king is threatened with capture, it is said to be in check, and the player must remove or evade the threat of capture immediately, such as by moving it away from the attacked square. If this cannot be done, the king is said to be in checkmate, resulting in a loss for the checkmated player. A player cannot make any move that places their own king in check. Despite this, the king can become a strong offensive piece in the endgame or, rarely, the middlegame.
In algebraic notation, the king is abbreviated by the letter K among English speakers. The white king starts the game on e1; the black king starts on e8. Unlike all other pieces, each player can have only one king, and the king is never removed from the board during the game.