From the game's encyclopedia
A Title is essentially a certificate of land ownership.
The smallest unit of land on the map is the Barony. Baronies are organized into larger areas called Counties. Counties De Jure belong to a Duchy, duchies to a Kingdom, and Kingdoms to an Empire.
Duchies, Kingdoms and Empires do not always have a Holder, but are Titles that can be Created by a ruler who controls enough of the corresponding territory.
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.
Title is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Title is essentially a certificate of land ownership." It covers Primary Title, Barony, County, Duchy, Kingdom and 4 more. The game also links it as Titles and Title's. 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.