A rookie card (often abbreviated RC) is the first card produced of a player or athlete that is a part of a nationally distributed, fully licensed sports card set. The RC designation extends to all cards produced that fit the definition in a single year, so it is possible for one athlete to have many different rookie cards.
Note that a rookie card is not necessarily always produced during a player's rookie season, as there may have been cards that fit the definition printed in previous years. It is generally agreed that to be a true rookie card, the card must be numbered as part of a product's base set. Thus, insert cards of any type are generally not considered to be rookie cards.
Rookie cards aren't necessarily worth more than other cards of the same athlete, as other factors like scarcity and condition can play large roles in their value. However, when other attributes are roughly equal, a rookie card will likely command a premium over another, non-rookie card thanks to its special status among collectors.