From the game's encyclopedia
The general populace of each County has an Opinion of the Character who holds their County. If the population is unhappy, they might start or join a Peasant Faction or Populist Faction, either of which can soon spread throughout an entire Realm.
The easiest way of controlling Popular Opinion is to ensure that the County Holder is of the same Culture and Faith as the population.
Popular Opinion from having a different Faith is modified by both the county's Faith Hostility and its Fervor, scaling linearly between 0 and 100 Fervor.
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.
Popular Opinion is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The general populace of each County has an Opinion of the Character who holds their County." 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.