The new league policy, put in place for the 2018 season, says that players are allowed to stay in their locker rooms for the anthem, but if they are on the field, they must "show the anthem and flag 'appropriate respect.' "
The league says teams can set their own rules for such a policy, but that teams would fine players on the field for the anthem who do not show "appropriate respect."
The NFLPA shared this statement about their grievance.
The beginnings of the national rancor over the issue started with the 2016 Packers-49ers preseason game where Milwaukee native Colin Kaepernick, then a quarterback with the 49ers, sat down during the anthem to protest about social issues, particularly police treatment of minorities.
Protests with players taking knees soon grew across the NFL the last two years, as the league policy at the time was to actively encourage but not require players to stand for the anthem as leagues as the NBA does.