What is offside in soccer, finally explained?

The rule in one breath: at the moment a teammate plays the ball forward, an attacker is in an offside position if any part of their head, body, or feet is in the opponent's half and closer to the goal line than both the ball and the second-to-last defender, the goalkeeper usually counting as one of the two. Being in the position is not itself an offense; the flag comes only if the player then gets involved, by playing the ball, obstructing an opponent, or profiting from the position.

The built-in exceptions people forget: you cannot be offside in your own half, or when receiving the ball directly from a throw-in, corner kick, or goal kick.

Why it exists: without it, teams would park a striker on the opposing goal line and boot long balls forever, which was killing the sport in the 1860s. The rule forces attackers to time runs behind the defense, which is the entire art form, and VAR's armpit-level offside calls are the modern price of measuring it to the centimeter.