From the game's encyclopedia
Men-at-Arms are professional Soldiers superior to Levies.
The player has direct control over the composition and size of their Men-at-Arms Regiments.
Different types of Men-at-Arms are good or bad in different types of Terrain. Each type also Counters another type.
Men-at-Arms can be Stationed in Holdings to increase their effectiveness.
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.
Men-at-Arms is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Men-at-Arms are professional Soldiers superior to Levies." It is part of Soldiers. It covers Skirmishers, Archers, Light Cavalry, Heavy Cavalry, Spearmen and 7 more. 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.