A play on college sports' "Redshirt" rule: they can't play, but they're still a part of the team.
Redshirt is another name that refers to underage girls (typically between the ages 16-17).
They're "a part of the team" = you can check them out, but "they cannot play" = you can't have sex with them because that's illegal.
"DAMN! Check out that Redshirt over there!"
"Yea, definitely going to be scouting her for the upcoming season"
"Yea, I wonder what her 40 (yard) time is looking like.."
"She's definitely going to be a starter one day."
"Bitch is going to the SEC with that talent."