What does listen is the key mean?

I think it means that if you listen, that is the answer, or "the key", to what is about to happen or what you should do. For Example:

Blue = listening

Purple = the outcome of listening

Black = normal writing (not important)

The teacher said, "You must fold the page sideways, then throw it across the room." So Jimmy folded the paper sideways and threw it across the room.