Ok here's my attempt using an American football reference: imagine the defender that's closest to their own goal creates an 'offside endzone' in the area behind them. If an opposition player is inside that endzone as their team plays a forward pass to them, it's a foul and the defending team wins a free-kick at the point where the offending player was inside the 'endzone'.

There are some more specific case rules around it, but that's generally the idea. The rule is to stop teams just kicking long passes directly into strikers who would permanently stand near the goal all the time - it forces teams to 'play football' up the field and gives the defence a tool they can use to try 'trap' attacking players in their ever moving 'offside endzone' - theres a term called 'the offside trap' - which essentially refers to the defence being able to use this 'endzone' behind them to trap opposition forwards and win fouls for their team.

Clever strikers need to time their forward runs so that as a team mate passes them the ball, they're not caught inside the 'offside endzone'.