From the game's encyclopedia
Allows to invest GoldGold in order to motivate the Peasant Faction in another Realm, if they have Counties with negative Popular Opinion.
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.
Foment Revolt is an entry of Crusader Kings III's in-game encyclopedia (The Fate of Iberia). The game explains it: "Allows to invest GoldGold in order to motivate the Peasant Faction in another Realm, if they have Counties with negative Popular Opinion." Patch 1.20.1.2, from game/common/game_concepts/03_fp2_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/03_fp2_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.