From the game's encyclopedia
Your Tributary Taxes consist of the Taxes that you receive from your Tributaries. The amount depends on each Tributary's Obligations.
From your Tributaries' point of view, these same taxes are called Suzerain Taxes to distinguish them from any Tributary Taxes they collect from their own Tributaries.
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.
Tributary Taxes is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Your Tributary Taxes consist of the Taxes that you receive from your Tributaries." It is part of Taxes. 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.