This was harder to articulate than I thought it would be😂 
So usually, if someone passes the ball to someone that’s behind enemy defense players, it’s an offside. Let’s say Messi is lurking a couple of feet behind the most defensive opposing player. If someone else passes the ball to him, he can’t catch the ball because he was offside when the pass was done. He’d have to be right beside the defensive player or in front of him at the time of the pass to be onside. Does that make any sense to you?