so quiet you could hear a pin drop

If it is so quiet you could hear a pin drop it means in the room you are in it is completely silent without a sound. In order to hear a pin drop, it would need to be as silent as possible because a pin dropping on the ground, even tile, is so faint that most people would not be able to hear it even in a very quiet room. This is usually said in an exaggerated way to explain just how quiet something is or possibly a situation.