When commenting with Cove, your readers will register and sign in using Ghost's in-built authentication system.
Ghost Members are subscribers of your blog and can receive newsletters as well as log in to your site. If you offer paid plans, members (once logged in) can also typically edit their payment options and cancel their subscriptions. (Each theme may serve this slightly differently.)
Ghost log ins are password-less and use "magic links" over email. A user will input their email address into a log in form and receive an email containing a link. Once clicking the link, that user is logged in to your site (not logged in to your Ghost admin, but the front-facing blog).
Even if your theme doesn't support memberships fully (ie: it doesn't have a payment form or a newsletter subscribe form), Cove works on any theme. You will add a log in form when you integrate Cove into your Ghost site.
Cove uses Ghost's password-less system to log members in for commenting. Once they have clicked the link and completed authentication, Cove recognises the user session and enables the comment form on your posts. Until they log out of your site, they will have access to Cove's commenting system.