Each NFL team carries a practice squad of players who train with the team during the week but are not on the active roster for games. These players can be signed away by other teams or promoted to the active roster at any point during the season. Teams use the practice squad to develop young players and keep depth on hand in case of injuries.
For fantasy football, practice squad players are not worth rostering in standard leagues since they cannot play in games while on the squad. They only become interesting when a team promotes them to the active roster, which often happens after an injury to a starter. A running back or wide receiver promoted from the practice squad can sometimes step into a big role right away.
In very deep leagues or dynasty formats, savvy managers watch practice squad moves for this reason. If a starting running back goes down, checking which back gets called up from the practice squad can help you find a waiver wire pickup before other managers notice.