From the game's encyclopedia
A Heritage is a Cultural Pillar that represents the common ancestry of several Cultures.
Cultures of the same Heritage generally have high Acceptance of one another.
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.
Heritage is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Heritage is a Cultural Pillar that represents the common ancestry of several Cultures." It is part of Cultural Pillar. 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.