This is a term used in most every sport at every level of competition, NFL football included, to refer to a first year player. In the National Football League, a first year player may be acquired through the NFL Draft or as a free-agent if they went undrafted. A rookie is not inexperienced in the sport, only at the level of play to which they have recently risen. Many NFL rookies have played their entire lives, including at college level. A rookie is not necessarily young, though this is usually the case. If a player has never played NFL football before, they are considered a rookie, despite their age. The slang usage of the term may refer to an individual who is inexperienced at the sport. For example, "You really are a rookie at football."
In the NFL, the rookies have special contract rules which limit how much a team can pay them as well as limiting the length of the contract. The most stringent rules surround players selected through the draft. Rookies signed as free agents because they went unselected or any other reason are far less limited in terms of salary and contract length.
Some top rookies include: Marshawn Lynch, Dan Marino, Jamal Lewis, Jim Brown, Ben Roethlisberger, Brian Cushing, Fred Taylor, Cam Newton, Lawrence Taylor, Barry Sanders, Randy Moss and Peyton Manning.