Offsides in football is when a player is standing in the wrong place when the ball is passed from one player of their team to another. It happens when the player is standing too close to the other team's goal when their teammate passes them the ball. This means it’s easier for the other team to catch the player and get the ball back, so it’s not allowed. The players need to stay behind the lines that are marked on the field, and only move forward when their teammate passes them the ball. That way everyone is spread out across the field and it’s fair.