A prospect is a player who hasn't yet made their Major League debut — they're in the minor league system, working their way up. In the hobby, prospect cards are a massive category because they represent the future stars of the game.
The most important prospect cards appear in Bowman products, which are specifically designed to feature pre-MLB players. A prospect's 1st Bowman card is often the single most valuable card in their entire catalog, even more than their eventual rookie card.
Collecting prospect cards is a speculative game — you're betting on potential. Most prospects won't become stars, but the ones who do can turn a $5 card into a $500 card seemingly overnight. It's high risk, high reward, and endlessly exciting.