From the game's encyclopedia
Some Holdings are fortified and need to be Besieged and Occupied in order to gain control of the County.
The Fort Level of such Fortified Holdings determines how long the Siege can be expected to last.
A high Fort Level will require Siege Weapons in order to be Besieged efficiently, although each Siege Weapon is only effective up to a certain Fort Level.
Increasing the Fort Level of a non-fortified Holding has no effect.
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.
Fort Level is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Some Holdings are fortified and need to be Besieged and Occupied in order to gain control of the County." 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.