A rookie is someone new or recently promoted to a higher level of competition or responsibility.
The word is most commonly used in American sports commentary to describe a team player in their first season - a rookie NFL quarterback, example, or a new player on the Ryder Cup team.
Where does the word rookie come from?
The etymological origin is uncertain. The OED suggests that it may be a corruption of the word recruit. It's first citation is from Rudyard Kipling's Barrack-Room Ballads (1892)So 'ark an' 'eed, you rookies, which is always grumblin' sore, referring to rookies in the sense of raw recruits to the British Army.