From the game's encyclopedia
Archer Cavalry are mounted Men-at-Arms equipped with ranged weapons.
While each kind of Archer Cavalry is unique, they generally Counter Heavy Infantry, and are in turn Countered by Archers equipped with crossbows.
Archer Cavalry are also often strong on flat Terrain, particularly Steppe.
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.
Archer Cavalry is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Archer Cavalry are mounted Men-at-Arms equipped with ranged weapons." It is part of Men-at-Arms. 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.