Blank slot cover
Sometimes called an expansion slot cover, a blank slot cover is a thin piece of metal that covers an opening in the back of a computer case where an unused expansion slot is located. The cover protects the inside of the case from foreign objects, maintains proper air flow, and eliminates electrical interference. When installing a new expansion card the blank slot cover must be removed first. The picture is an example of a blank slot cover, next to two expansion cards.
Tip: With screwless cases, you may not have a screw holding in each blank slot cover. Instead, the blank slot covers are held in place with one thumb screw attached to a metal holder that can be removed after the thumb screw is removed.