In football / soccer, being offside is receiving the ball in the goalkeeper's area

Explanation

You are in an offside position if, at the moment the ball is played by a teammate, any part of your body other than your arms is in the opponents' half and closer to the goal line than both the ball and the second-last opponent (usually a defender, with the goalkeeper last). Being in that position alone is not an offense, you are only penalized if you then take part in active play, for example by receiving the ball.