Redshirt is a designation for a players eligibility year, for example redshirt freshman means that he is a sophmore, but still allowed one more year to play.
A redshirt year is declared after the end of the championship season and the player cannot enter the game in any circumstance. The only exception is for injury, and that still is not determined until the NCAA approves it.
A player may be told he may be redshirted, and called in, perhaps needed to replace for injured players. Some players are given options, some are told.