From the game's encyclopedia
A Realm's Laws are what determine how that Realm is structured.
The most fundamental of these is the Realm's Government Authority, as those Laws determine how Lieges and Vassals interact on a regular basis.
Though less impactful on the day-to-day life of Rulers in a Realm, Succession Laws and Title Laws are still very important to consider for Succession.
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.
Law is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Realm's Laws are what determine how that Realm is structured." It covers Succession Law, Head of Faith Succession Law, Title Law, Church Authority, Government Authority and 3 more. The game also links it as Laws. 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.