This is a dummy discovery service. Normally this page isn't accessed directly. A service provider (SP) can redirect the client here according to the idp discovery protocol to have the client select an idp to use to authenticate.
The Stub Idp is part of the Sustainsys.Saml2 open source package for ASP.NET.
The SAML messages sent by the Stub Idp will be signed with this certificate. To validate the responses you need to download it and configure your service provider to trust it.
Metadata for the Stub Idp is available at /Metadata or in a browser friendly version version (with a content type that makes the browser display the entire XML). There is also federation metadata, with this idp as the only federation member available (browser friendly version).
The Stub idp contains a stub idp discovery service that can be used when testing federations.
The stub idp also features single logout support.