Here's the breakdown:

* You exert a downward force on the floor: Your weight (due to gravity) presses down on the floor.

* The floor exerts an equal and opposite force on you: This is the upward force you feel, keeping you from falling through the floor.

This is why you can stand on a floor without sinking! The floor is pushing back on you with the same force that you're pushing down on it.

This principle applies to all objects that are in contact with each other. It's why you can push a wall, and the wall pushes back on you. It's also why you can jump – you push down on the ground, and the ground pushes back up on you, launching you into the air.