rroque,
"Redshirt" is a term that doesn't appear in the NCAA rule book.
It is used in various ways.
When a player says, "I'm going to redshirt this year" it usually means he knows in advance he won't compete during a season, such as when he is recovering from an injury or is required to sit out after transferring from one D1 to another.
When a player says, "The coach might redshirt me," it usually means the player is healthy and eligible but is not expected to play, usually because the coach wants to save the year of eligibility for a season in which the player might make a bigger contribution to the team. An example of what RJM was talking about when he mentioned one player blocking another would be when the team's starting shortstop is a senior and the heir apparent to the position is a freshman. The freshman might be redshirted so he can still have his full eligibility remaining after the senior graduates.
Sometimes after a player goes through a season without playing, he says, "I redshirted last spring," and you don't know if it was part of a plan to use him later or if he just wasn't given any playing time.
Sometimes a school can obtain a waiver for a player who suffers a season-ending injury early in the season and not have that season charged against his four years of eligibility. Such a player might say, "I got a redshirt year after getting hurt."
After a player has sat out a year, "redshirt" is used as an adjective before his class name to explain the mismatch between his academic year and his eligibility year. A redshirt sophomore is a player who is in his third year of college but only his second year of competition.
Does this help?
I thought it was also possible for a redshirted player to also apply for a medical redshirt later. For example, incoming freshman is not going to have a chance to play, so he is redshirted his freshman year. His RS FR year (sophomore in school) he becomes a stud and starts his RS FR, RS SO & RS JR years. He dosn't go pro and goes back for his senior year as a post graduate student and is injured in the fall (call it TJ for the sake of argument) and applies for a medical redshirt for his RS SR year. The NCAA may or may not grant it, allowing him to come back again.
I'm not positive this is the case, but I do believe I've seen it.