From the game's encyclopedia
Characters gain Stress when taking actions that conflict with their Personality Traits.
Characters who gain enough Stress will also gain a Stress Level, causing various negative effects on their mental health that persist until they manage to reduce their Stress.
A straight-forward way to relieve Stress is to partake in Activities such as a Hunt.
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.
Stress is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Characters gain Stress when taking actions that conflict with their Personality Traits." The game also links it as Stress Loss. 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.