From the game's encyclopedia
Spearmen are Men-at-Arms who fight on foot with various polearms.
While each kind of Spearman is unique, they generally Counter Heavy Cavalry and Light Cavalry, and are in turn Countered by Heavy Infantry.
Spearmen are also often strong on hilly and mountainous 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.
Spearmen is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Spearmen are Men-at-Arms who fight on foot with various polearms." 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.