There are many actions that a Ruler can make in Crusader Kings III that are consider Tyrannical. Tyrannical actions are those against the law or religious doctrine. There are many examples of tyrannical actions such as, Imprisoning or Revoking Titles off someone who has committed no Crime is considered Tyrannical. Be aware that forcing additional obligations from your Vassals while modifying a Feudal Contract without having a fair trade or using a hook will also result in an act of Tyranny.
Tyranny acts as a blanket Opinion modifier for all characters in the Realm.