The client portal is where your client will see everything you do for them. Their galleries, invoices, questionnaires, contracts, and more will all be available in one place in their client portal.
If a client has more than one shoot with you, they will see a shoot chooser as the landing page on their client portal, and they can select which shoot they'd like to view the details for.
To see what a client's portal will look like, go to the Clients & Contacts page of any Lead or Shoot. Under the contact, click the "View as Client" button, which will redirect you to the client portal and show it to you as if you were logged in as that client.
Alternatively, when you are editing a contact, click the "User Actions" in the bottom-left corner of the modal, and click the "View as Client" button.
Note: If your browser has popups blocked, clicking the "View as Client" button may not open up the page. Try turning off your pop-up blocker and click the button again.