Tic-tac-toe is a simple two-player game played on a 3x3 grid. Players take turns marking a square with their symbol, either an "X" or an "O." The goal is to be the first to get three of their symbols in a row, either horizontally, vertically, or diagonally. The game can end in one of three ways: one player wins by forming a line of three symbols, the other player wins, or the game results in a draw if all squares are filled without a winner. Tic-tac-toe is often used to teach basic strategy and critical thinking skills.