From the game's encyclopedia
Taxes are the most important monthly source of Gold Income for Rulers.
Every Holding a ruler has provides them with some Domain Taxes every month. In addition, most Vassals have an Obligation to pay a portion of their own collected Taxes to their Liege in the form of Liege Taxes.
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.
Taxes is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Taxes are the most important monthly source of Gold Income for Rulers." It covers Vassal Taxes, Liege Taxes, Tributary Taxes and Suzerain Taxes. The game also links it as Tax and Taxation. 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.