From the game's encyclopedia
Perks are unlockable boons that provide many useful and interesting effects. They are unlocked by selecting a Focus within a Lifestyle and spending the associated form of Experience (e.g. Intrigue Experience if you wish to unlock an Intrigue Lifestyle Perk.)
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.
Perk is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Perks are unlockable boons that provide many useful and interesting effects." The game also links it as Perks. 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.