A DummyIDP app lets you emulate your customer's identity provider.Docs
SAML IDP SettingsDocs
These are SAML settings that identity providers (DummyIDP, in this case) assign. Normally, they'll come from your customer's Okta/Google/Microsoft/etc instead. You need to put these into your application's SAML settings.
SAML SP SettingsDocs
These are SAML settings assigned by the service provider ("SP"), i.e. your application. You need to copy those settings from your application into here.
SCIM SettingsDocs
Settings for directory syncing over SCIM. Optional.
UsersDocs
You can simulate SAML logins from this list of users. They'll be synced over SCIM if you've configured it.