Each team has one injured reserve (IR) slot. The player has to be placed on IR in the NFL to qualify. You can then hold that player, essentially as a 17th player, until keeper declaration day the following season. At that time, you could use the player as one of your three keepers, or you could throw him back into the draft pool. He could also be traded to another team in the offseason to be a keeper, or even during the season he was put on IR, but only if the receiving team's IR slot is open. Also, when an owner places a player on IR, he may then make an immediate pickup of another player.