From the game's encyclopedia
Herder realms are removed from all aspects of rulership. They do not command Armies, own Buildings, create Alliances, or declare Wars. When Nomads Migrate a Holding, herders will slowly replenish the local County Fertility by virtue of their smaller flock.
They are intimidated by proper Nomad Rulers and will immediately surrender their land when challenged.
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.
Herder is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Herder realms are removed from all aspects of rulership." It is part of Government. The game also links it as Herders. Patch 1.20.1.2, from game/common/game_concepts/mpo_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/mpo_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.