Per reporters at the practice, Belichick had players wear practice jerseys with no numbers or names on the back of them. That's something Belichick did with the New England Patriots, until the NFL made him stop. The goal is for players to 'earn' their jerseys:

The North Carolina Tar Heels took the practice field on Wednesday afternoon for the second time under new head coach Bill Belichick—and they did so with an interesting twist. The Tar Heels' jerseys they were wearing didn't have either numbers or names on the back of them.