What s single sign on and how to implement in my application

1) I want to Implement single sign on my application. i have no idea about the single sign and how to do it.this is one requirement of my project.

2) i refer some website they are explain the single sign on concept but i don't understand how to do it.

 

I have below code to do it but i have no idea to where is implement this code.

 

 

void Session_Start(object sender, EventArgs e)
{
//// Code that runs when a new session is started
Session["LoggedUserID"] = null;
string LOGIN_USER = System.Security.Principal.WindowsIdentity.GetCurrent().Name;
if (LOGIN_USER != null && LOGIN_USER != "" && LOGIN_USER != string.Empty)
{
LOGIN_USER = LOGIN_USER.Substring(LOGIN_USER.IndexOf("\\") + 1);
Session["LoggedUserID"] = LOGIN_USER;
}

}