Understand Federated Login
The federated login will allow you to authenticate the customers of a portal by using their Google or Facebook account. The advantage of using federated login is that your customers will not need to create an account to access the customer portal. Instead Google or Facebook will act as an identity provider and verify the identity of the customer.
Let's assume an organization called 'Zylker'. They have a customer portal called 'Subscriber registration' in which the new customers have to fill up the data required for the subscription. To make it easier for the customers to subscribe, Zylker wants them to be able to use their Google accounts. Zylker can achieve this by using the Federated login option in customer portal. When the customer visits the Customer portal they will be displayed an option to access it by using their google account. Once they enter the correct credentials, then their account will be authenticated and they can access the customer portal.