A controlled ground area is a patch of ground where the UAS operator ensures that only persons involved in the operation are present during the flight. This is achieved practically: with cordons, marshals and announcements that keep random passers-by from entering the area.
It is the core ground-risk mitigation of the standard scenarios (STS-01 and STS-02) and of specific-category operations: the risk to bystanders is controlled simply by keeping them out from under the flight. If the area cannot be kept "clean", the scenario's conditions are not met.