https://www.sbnation.com/college-fo...a-football-grayshirt-blueshirt-redshirt-rules
"Athletes who grayshirt are allowed to enroll as students. They go to class for the first semester as part-time students, either at the school or at a junior college, without starting their eligibility clocks. Then they begin as full-time students on scholarship.
Grayshirts have commonly been used by programs that oversign, which means not having enough roster spots for all of their commits. Sometimes, coaches are up front about grayshirting from the beginning. However, other times, they will force grayshirts on athletes at the last minute, leaving them unable to find schools that will allow them to play immediately.
Last year,
Louisville coach Bobby Petrino was criticized for giving one player a surprise grayshirt right before National Signing Day, after the player had been committed as a non-grayshirt for a year. Many other programs have done the same thing.
Sometimes, grayshirts can be revoked and turned into regular offers due to unforeseen roster changes. If there is a surprise transfer during the offseason, a player who planned on grayshirting could be added to the roster as a normal signee."