From the game's encyclopedia
A Modifier is an effect that changes a value over a longer period of time. It may for example change the value of a character's Skills, or the Success Chance of a Scheme.
Modifiers typically appear for Characters, Schemes or Counties.
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.
Modifier is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Modifier is an effect that changes a value over a longer period of time." It covers County Modifier and Character Modifier. The game also links it as Modifiers. 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.