From the game's encyclopedia
High Tribal Authority is the second-highest level of Tribal Authority. Vassals have a -20 Opinion penalty toward their Liege.
At this level, as well as the ability to Imprison Characters, the liege may Revoke Titles and Retract a vassal's vassals.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
High Tribal Authority is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "High Tribal Authority is the second-highest level of Tribal Authority." It is part of Tribal Authority. The game also links it as High. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.