Game concept
Supply
The game also links it as Supply State, Supplies, Supplied.
From the game's encyclopedia
Supply is the amount of food and equipment an Army carries. Armies will begin losing Supply when their size goes above the Supply Limit of their current location.
Armies that are running low on Supplies will suffer an Advantage penalty in Battle, and when they run out completely they will begin suffering from Attrition.
Supply increases over time as long as the Army remains below the Supply Limit in friendly 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.
Supply is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Supply is the amount of food and equipment an Army carries." The game also links it as Supply State, Supplies and Supplied. 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.