From the game's encyclopedia
Tribal realms have stunted county Development, cannot advance past Tribal Era Innovations, cannot construct new Holdings, and are usually unable to change their Succession Law.
However, they can make use of Prestige rather than Gold to build their realms and hire Men-at-Arms, and have access to more Casus Belli compared to Feudal Realms. Non-Tribal armies suffer a Supply Limit penalty in hostile Tribal territory.
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.
Tribal is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Tribal realms have stunted county Development, cannot advance past Tribal Era Innovations, cannot construct new Holdings, and are usually unable to change their Succession Law." It is part of Government. The game also links it as Tribal Government. 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.