From the game's encyclopedia
County Control measures the grip that the government has on the population of a County. It is crucial especially for collecting Taxes from the Holdings.
When a County changes hands through War, Control goes down drastically, which is a good reason to vassalize the current Holder instead (if the Casus Belli allows). County Control can also be damaged by Sieges and Peasant Uprisings.
Low Control may lead to County Corruption, and the primary way of increasing Control is a Councillor Task available to your Marshal.
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.
County Control is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "County Control measures the grip that the government has on the population of a County." The game also links it as Control. 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.