What exactly does UrlReferrer mean?

which means UrlReferrer ( Request.UrlReferrerand Request.UrlReferrer.OriginalString)?
I wrote these codes:

    if (Request.UrlReferrer != null)
    {
        User_Info.Add(!string.IsNullOrEmpty(Request.UrlReferrer.OriginalString) ? Request.UrlReferrer.OriginalString : string.Empty);//6:UrlReferrer
    }
    else
    {
        User_Info.Add(string.Empty);
    }

I just know Querystring ReturnUrl when a user tries to enter the administration area.
therefore, in web.config <authentication mode="Forms">
and now we have the server code UrlReferrer.
I want to collect some information about my users of my website, such as User-Agent, User-Browser, Session-ID, etc.

but I really don't know what UrlReferrer does, and it is always zero (I think due to misuse)
could you give me a simple example to understand.

Thank you for the attention