What is Single Sign-on?

[DunbarPappy®ϟϟ]

I think you want to use one site's account (which you authenticate with), and have that authentication be invoked to other sites. amirite?
Have a go with https://en.wikipedia.org/wiki/Oauth site to see if it fits your needs, but other 'cross-site' authentication methods are available.
The site lists websites that utilize this method, plus some diagrams to help understand how it works.