Authentik is an open-source identity provider that Nest uses to keep track of users across Nest services, from this wiki to the Nest server itself. All Nest users will have an Authentik account. You may also see it referred to as Nest Identity.
You can access your Authentik account at https://identity.hackclub.app. The login is your Nest username and password (see Quickstart for how to sign up), or your Slack account if you've linked it. There's not much to see here, but you can change some settings such as your password, link your Slack account, and add multi-factor authentication.