SSO is a feature available starting on our Enhanced Plan that allows your customers to be automatically signed into UserVoice if they have already logged in on your website. In essence, when someone logs into your website, those credentials can be passed in the form of a token to your UserVoice site. This way, if they perform an action on your UserVoice site which would normally require them to submit an email address (such as submitting a ticket or an idea) they won't have to sign in again (or they will be redirected to your site to login once).
Please note that this is by no means trivial to set up. Since it involves passing information from your site backend, it requires some coding, and should only be done by someone familiar with the programming language for your site backend.
We've written this guide for developers who intend to set it up.