From the game's encyclopedia
Culture represents the language and customs of a Character or County.
Characters of different Cultures will have lower Opinions of each other. This penalty can be reduced by increasing Cultural Acceptance.
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.
Culture is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Culture represents the language and customs of a Character or County." It covers Cultural Acceptance, Cultural Pillar, Cultural Tradition, Culture Intermingling, Hybrid Culture and 1 more. The game also links it as Cultures, Cultural and Culture's. 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.