From the game's encyclopedia
The Coronation is an Activity wherein a Ruler invites Vassals, peers, and Guests from far and wide to witness their ascension to the throne.
The Coronation comes in two forms, a regular Coronation and an Anointment, with the latter providing better rewards, but at greater costs.
Anticipated rewards:
• 
Crowned Sovereign or Anointed Sovereign Law
• While the Coronation exists, 
Factions cannot Press Demands
Factions and other foes will harm the Coronation's Magnificence
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.
Coronation is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Coronation is an Activity wherein a Ruler invites Vassals, peers, and Guests from far and wide to witness their ascension to the throne." It is part of Activity. It covers Magnificence, Oath, Supporter, Detractor and Anointment. The game also links it as Coronations. Patch 1.20.1.2, from game/common/game_concepts/ach_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/ach_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.