Floor registers are used to cover up the vent opening in your wall, ceiling or floor. They also help to control the air flow to the room either by limiting, redirecting, or stopping air flow altogether. They will help save on your energy costs as well as keep your room temperature comfortable.
The displayed measurement of the floor register refers to the size opening of your vent. So, for example, if the hole in your floor made by the vent ducts is 4" x 8", then you would choose a 4" x 8" Floor Register.