Blank slot cover
Sometimes called an expansion slot cover, a blank slot cover is a thin piece of metal covering an opening in the back of a computer case for an expansion card. 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 shows a blank slot cover, next to two expansion cards.
With screwless cases, you may not have a screw to secure each blank slot cover in place. 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.
A blank slot cover may also be called an adapter plate, slot blocker, or slot shield.