From the game's encyclopedia
A Follower is an Unlanded character who accompanies an Adventurer.
Mechanically, they are equivalent to Courtiers, though they are under less obligation to show respect or hang around if they dislike their leader.
The collective Opinion of a character's Followers determine their Camp Temperament.
Followers can:
• Serve as Officers
• Serve in Armies as Commanders or Knights
• Let their Adventurer press their Claims in War
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.
Follower is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Follower is an Unlanded character who accompanies an Adventurer." The game also links it as Followers and Following. Patch 1.20.1.2, from game/common/game_concepts/07_ep3_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/07_ep3_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.