From the game's encyclopedia
Light Cavalry are mounted Men-at-Arms who excel at scouting and pursuing the enemy.
While each kind of Light Cavalry is unique, they generally Counter Archers, and are in turn Countered by Spearmen.
Light Cavalry are also often strong on flat Terrain.
In comparison with Heavy Cavalry, they are usually better at Pursuit and Screen.
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.
Light Cavalry is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Light Cavalry are mounted Men-at-Arms who excel at scouting and pursuing the enemy." 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.