From the game's encyclopedia
Skirmishers are Men-at-Arms who are lightly armed and skilled in harassing the enemy.
While each kind of Skirmisher is unique, they generally Counter Heavy Infantry, and are in turn Countered by Archers.
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.
Skirmishers is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Skirmishers are Men-at-Arms who are lightly armed and skilled in harassing the enemy." It is part of Men-at-Arms. The game also links it as Skirmisher. 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.