A rookie card is a player's first card after making their Major League debut — and in the hobby, rookies are king. These cards are typically stamped with an "RC" designation on the front or back, appearing in the same year as the debut or sometimes the following year.
Star players can have dozens of different rookie cards across multiple products, and collectors will inevitably decide which one is the most important — and most valuable. The consensus "top" rookie card usually commands a significant premium over the others.
Rookie cards are the foundation of player collecting and prospecting. They're almost always more valuable than any card released later in a player's career. If you're building a collection around a specific player, their rookie card is the centerpiece.