This extensions makes the permission flag on a contact's relationship work as a true ACL. In core CiviCRM that flag only allows the user to see the contact's dashboard. However, in many cases it is a useful mechanism to give people permission to view contact records and search for contacts.
Contacts also get 'transitive permissions' - if you give, for example, a secretary permission over an organisation they will have permission over anyone that organisation has permissions over. Transitive permissions go only one step & only where the first contact is a Household or Organisation