What a player is injured, or just the team chooses to give him the "red-shirt" as is common for freshman, they actually wear a red shirt for practice. They can't play in the games but can practice with the team. This preserves their year of eligibility. They can only use it once, but can apply for exemptions and be able to play extra years, but that usually only happens in a case where a player loses most of a season, or two, from eligibility.