A penny sleeve is the most basic — and most essential — card protection accessory in the hobby. It's a thin, soft, clear plastic sleeve that slides over a card to shield it from dust, fingerprints, and minor surface contact. They got their name because they used to cost about a penny each.
Penny sleeves are the first line of defense for every card you care about. The standard practice is to sleeve the card, then place the sleeved card into a toploader or One-Touch for rigid protection. Sleeve first, always.
They're dirt cheap, they come in bulk packs of hundreds, and there's no excuse not to use them. Every collector should have a stash of penny sleeves within arm's reach at all times.