Basically a rule which means a player can’t just stand and wait by the opposition’s goal for an opportunity to score; if a player is actively involved in the progression of the ball down the pitch, they must always be behind one of two things:

a) The ball

b) The opposition’s second deepest lying player (the deepest lying player almost always being the goalkeeper, but doesn’t have to be)

It’s a bit more complex than that but this is the explanation that made me understand when I was first getting into football :)