Followers are a type of unit. They represent characters, and occasionally locations (eg. 3 Wyrding Stones) and objects from the world of Runeterra at various points in time that are not Champions in League of Legends in the state represented by the card (eg. 3 Senna, Sentinel of Light and 7 Yone, Windchaser are and when they were fully human.)
A player can own duplicates of a follower, and unlike champion cards a player can have multiple duplicate follower cards on the board at the same time.
Some follower cards will only appear if a certain champion is on the board or if certain criteria is met.
In normal game mode, these cards can be from different regions, or have duplicates. Just like other cards, a player can have at most 3 duplicates of one follower.