What is a rookie?
A rookie is a talented newcomer (novice, beginner) who has risen to the level of a professional athlete with no experience in professional sport and is competing in his or her first professional season. The term rookie probably spilled over to us from the USA. There, the term can be found in sports, such as American football, basketball and other sports. It is assumed that it is a variation of the word "recruit", which would make the rookie a "newly recruited" player.
Very often, a player's first collector's picture does not coincide with the year of his rookie season. In the USA, for example, in American football, special "draft" picture series are released every year with all the rookies. In the draft, professional clubs sign the most talented college players and officially start their professional careers. When a player grows into a successful star over the course of his career, his rookie cards experience a huge price increase on the collectors' market.