contains all, you evaluate all values of the list on the right side of the operator against the list on the left. It only evaluates to true if the list on the left includes all elements from the list on the right.
p.additional_services contains all FreeServices
You have a taxonomy defined, which lists all the free services your organization offers to clients. When signing a client up for additional, paid, services, you want to make sure all free services are also included in the deal.
The expression in the example will only evaluate to true if that is the case.