A floor is the surface at the very bottom, or base, of a room. When you lie on your back on the floor, you are looking up at the ceiling.
Your living room floor might be covered with carpet, while the floor of your tent is probably canvas or nylon. While a floor is the bottom of a structure, it's also the lowest surface of the ocean: "Crabs scuttle across the ocean floor." If you live in a dorm or stay in a hotel, your floor is the level of the building where your room is located. Informally, if something floors you, it confounds or confuses you.