Some monsters in Diablo 4, especially Bosses and Elites, have Crowd Control attacks that can, among other things, Freeze, Slow, or Stun players. Instead of being forced to dangerously wait out the duration of the Crowd Control effect, players can use a Skill or item to trigger Unstoppable Status, immediately freeing them[1]:
"Many Skills in Diablo 4 grant Unstoppable Status. This allows you to break out of any control impairing effect, so if you're Stunned, Blinded, Slowed -- you can break out of it and are granted immunity for the duration of the effect. In the case of our Barbarian friend, who doesn't want to deal with this Bone Storm any more, he can use Rallying Cry, becoming Unstoppable and granting his allies Unstoppable as well and immediately dish out more pain to the Tomb Lord."
Xiphoid has been playing Diablo since the original's release in 1997. He made one of the largest Age of Empires sites in the late-'90s and early-'00s. Now, he runs MythicDrop.com, a premier Diablo 4 fansite.