The SAMLRequest is the SAML logout request encoded using the HTTP-redirect binding. The logout request includes an issuer which identifies the service provider that sent the logout request.
If you use the SAML high-level API the processing of the logout request including decoding the message etc is handled automatically. The ExampleIdentityProvider and ExampleServiceProvider projects demonstrate this.
If you use the low-level API then you can call SingleLogoutService.ReceiveLogoutMessageByHTTPRedirect which will receive and decode the logout request. The low-level API SAML2IdentityProvider project demonstrates how to make this call.
However, you are best to use the high-level API if possible as it is easier to use and requires less code in your application.