From the game's encyclopedia
Under the Feudal Government Form, a Liege gives their Vassals Land to govern in exchange for Levies and Taxes, regulated by individual Vassal Contract between Vassal and Liege.
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.
Feudal is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Under the Feudal Government Form, a Liege gives their Vassals Land to govern in exchange for Levies and Taxes, regulated by individual Vassal Contract between Vassal and Liege." It is part of Government. The game also links it as Feudal Government and Feudalism. 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.