Some fantasy football leagues will give managers the option of sending injured players to the Injured Reserve for a designated number of weeks. Another player can be added to the fantasy team because the IR player does not count against the player limit. If the player returns to the line up before the designated number of weeks has passed, the fantasy owner cannot earn points for that player, until that players is activated and removed from the IR. NOTE: This is different from the IR in the NFL, which players cannot be activated from.
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 also an NFL label for players who will not play during the current season due to injury.
A roster spot that is used to keep an injured player "reserved" from the rest of the league. Typically, a player on injured reserve is on a team's roster but not allowed to be placed in the active lineup.