From the game's encyclopedia
A Holy Site is a County which contains a place of religious significance to a Faith. Holy Sites held by a Character of the correct Faith will provide an effect to everyone of that Faith, and can also allow the creation of Special Buildings or Holy Orders. Each Faith has a different set of Holy Sites.
An ordinary Holy Site gives no global faith benefits, only local ones. A Faith's most revered sites are instead designated as Eminent Holy Sites. A Faith can only have a limited number of each.
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.
Holy Site is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Holy Site is a County which contains a place of religious significance to a Faith." It covers Enshrined, Sanctified, Eminent Holy Site and Holy Relic. The game also links it as Holy Sites. 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.