From the game's encyclopedia
Your Domain Income consists of the Taxes that you collect from your own Domain. The amount collected depends on the local Control and Development levels.
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.
Domain Income is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Your Domain Income consists of the Taxes that you collect from your own Domain." The game also links it as Domain Taxes. 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.