An option in some leagues, an injured player can be sent to the IR for a certain number of weeks. The player cannot return to the active lineup and does not earn the owner any points until that number of weeks passed; however another player can be added to the team since the IR player doesn't count against the player limit. Injured Reserve is [..]

Not applicable for fantasy football as a roster spot, but something to note. In the NFL, once a player is placed on injured reserve, he cannot participate in another game for the remainder of the season.