Followers are a type of unit. They represent characters, and occasionally locations (eg. 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. Senna, Sentinel of Light and 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.