What does mappedBy actually do in a bidirectional @OneToMany/@ManyToOne relationship?

It marks the @OneToMany side as the inverse (non-owning) side, telling Hibernate the foreign key already lives on the other (@ManyToOne) side's table — so Hibernate doesn't create a separate join table for this relationship. Changes must be made (and persisted) via the owning side; mutating only the inverse side's collection without updating the owning side won't persist.