From the game's encyclopedia
A Conqueror wants to expand their Realm through War, at any cost. They will invest all of their Gold into the War Chest or to buy Men-at-Arms.
They tend to declare many more Wars than normal, and are rather untrustworthy Allies. They forsake most frivolous pleasures in life, such as hosting or going to Activities and sponsoring Inspirations — to save Gold for crushing their enemies. They will only use Activities to gain enough Prestige and Piety to go to War.
Any non-Cautious Ruler has the potential to become a Conqueror, but normally there are no more than a handful of Rulers worldwide with this archetype.
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.
Conqueror is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Conqueror wants to expand their Realm through War, at any cost." It is part of Economic Archetype. The game also links it as Conquerors. 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.