With contains only, you check that the list on the left only contains values from the list, which could be a taxonomy, on the right.
p is Person
p.selected_services contains only free_services
You have a taxonomy defined, which lists all the free services your organization offers to clients. When onboarding new clients, you want to check whether the clients have only selected the free services or if you need to charged them for additional paid services.
The expression in the example will only evaluate to true if a client has only selected free services, and false if one or more paid services have been chosen.