What is Single Sign-on?

[Binnicker]

How to get it for my websites???

[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.

[savage]

Single sign-on is a way of allowing your users to login in your multiple set of websites with only one identity. This makes the identity management of your users easy for website owners but also for the users to manage their identity in the online world. Now, a days websites are implementing social single sign on where once user creates an account with one website with an existing ID provider like facebook, google, linkedin, myspace or others, he can not only log into that website using his/her existing ID but also all other website. This is particularly challenging to implement. Many solutions are readily available in the market. I personally tried http://www.loginradius.com and found their solution very well suited for my websites