From the game's encyclopedia
Archers are Men-at-Arms who fight on foot with ranged weapons.
While each kind of Archer is unique, they generally Counter Skirmishers, and are in turn Countered by Heavy Cavalry and Light Cavalry.
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.
Archers is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Archers are Men-at-Arms who fight on foot with ranged weapons." It is part of Men-at-Arms. The game also links it as Archer. 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.