Game concept
Quality
From the game's encyclopedia
Quality gauges the power of an Army relative to its number of Soldiers.
An Army that only contains Knights and Men-at-Arms will be of high Quality while an Army containing a large number of Levies will be of lower Quality.
This means that a smaller Army with a high Quality will punch well above their weight — carefully consider both the amount of Soldiers and Quality before engaging the enemy!
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.
Quality is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Quality gauges the power of an Army relative to its number of Soldiers." It is part of Soldiers. 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.