A card sleeve — commonly called a penny sleeve — is a thin, clear plastic sheath that slides over a card to protect it from surface scratches, dust, and fingerprints. It's the most basic and essential form of card protection in the hobby.
Penny sleeves got their name because they originally cost about a penny each. They're still incredibly affordable, and every serious collector goes through them by the hundreds. The standard practice is to sleeve a card first, then place it in a toploader or magnetic holder for additional protection.
If you're not sleeving your cards, you're leaving them exposed to damage that could tank their condition and value. Sleeves are cheap insurance — use them on everything.