From the game's encyclopedia
A Character's Traits describe their personality, quirks, deeds and state of being.
Traits can have many effects, but many affect the Skills of a Character.
Personality Traits also greatly affect how characters can be expected to behave. Acting against your own Personality causes Stress.
Some Traits are Congenital and can be inherited by offspring.
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.
Trait is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Character's Traits describe their personality, quirks, deeds and state of being." It covers Trait Path, Commander Trait, Education Trait, Personality Trait, Lifestyle Trait and 5 more. The game also links it as Traits. 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.