From the game's encyclopedia
Peasant Militias are Men-at-Arms who are armed with pitchforks and spears.
While each kind of Militia is unique, they generally Counter Heavy Cavalry and Light Cavalry, and are in turn Countered by Heavy Infantry.
Peasant Militias are strong in mountainous and fertile Terrain.
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.
Peasant Militia is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Peasant Militias are Men-at-Arms who are armed with pitchforks and spears." 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.