From the game's encyclopedia
Court Positions are specialized roles Courtiers and Vassals can be granted in order to aid their Liege.
The benefits you gain from a Court Positions are dictated by the holder's Aptitude for the position.
Giving out Court Positions is free, but you have to pay a monthly salary in Gold, and revoking them costs Prestige.
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.
Court Position is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Court Positions are specialized roles Courtiers and Vassals can be granted in order to aid their Liege." It covers Master Assassin and Court Position Task. The game also links it as Court Positions. 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.