Offside causes more pub arguments than any other rule in football. Here it is without the legalese: a player is offside if they are nearer to the opponent's goal than both the ball and the second-last opponent when the ball is played to them.
The simple picture
Imagine freezing the moment a pass is kicked. Draw a line through the second-last defender (usually the last outfield player, since the goalkeeper is typically last). If your teammate is beyond that line — closer to goal — and the ball is played to them, the flag goes up.
When you CAN'T be offside
Four big exceptions: you cannot be offside from a goal kick, a throw-in, or a corner. And you cannot be offside in your own half, no matter where the defenders are. Level with the defender? You are onside — the attacker gets the benefit.
Why VAR made it controversial
VAR measures offside to the centimetre, so goals are now disallowed for a shoulder or a toe being beyond the line. It is technically correct and emotionally brutal — which is why every marginal call sparks a week of debate.
The one thing to remember
Offside is judged when the ball is PLAYED, not when it is received. A player can sprint past the defence after the pass is kicked and still be onside. Watch the kicker's foot, not the runner — that is where the decision is made.