From the game's encyclopedia
Pillars are qualities that reflect the origins of a Culture.
There are 5 distinct Cultural Pillars:
• Ethos
• Heritage
• Language
• Aesthetics
Of these, Ethos, Language and Martial Customs can be changed by the Culture Head through Reformation.
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.
Cultural Pillar is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Pillars are qualities that reflect the origins of a Culture." It is part of Culture. It covers Heritage, Ethos and Martial Custom. The game also links it as Cultural Pillars. 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.