It is just like an API call which is used for integrating data with any system you might have. It posts data like contact info, list ID etc. to the URL entered in this field. Whenever a new contact is added or an existing one is modified, deleted or there is a change in subscription ( subscribed to unsubscribed or vice versa) etc, all the details of the contact (default and custom fields) will be sent to the URL stored inside the POST URL box. 

Sample JSON format

{"subscriptions":[{"list_id":5,"list":"TestList","status":"Subscribed"},{"list_id":21,"list":"Testlist1","status":"Subscribed"}],"changed_attributed":{"status":1,"active":true},"status_code":3,"active":false,"list":"Test List","list_id":5,"status":"Complained","contact":{"id":31280,"account_id":1,"email":"","first_name":"tets","last_name":"test","phone":null,"fax":null,"created_at":"Wed, 06 Mar 2013 19:00:06 EST -05:00","updated_at":"Wed, 06 Mar 2013 19:00:06 EST -05:00","domain_name_id":2483,"custom_field_1":null,"custom_field_2":null,"custom_field_4":null,"custom_field_3":null}}