A toploader is a small plastic shell that is usually used to hold collectible cards, such as baseball cards, Pokemon TCG cards, Magic TCG cards, etc. It uses the same concept as a penny sleeve, but is larger so as to encompass an already sleeved card. Once sleeved, the card is simply slipped into the toploader from the top (hence the name "top loader".) Usually, cards that are of collector's value are placed into toploaders after they are sleeved for greater protection (flipping through a binder can wear down cards over time.)
Toploaders are also available for comic books, and are just big enough to encompass a sleeved comic book of standard size. The principle is exactly the same; the comic book is slipped through an opening in the top of the shell much like a sleeve.
Brevity Quest 2022