AuthnRequest is a term borrowed from the SAMLTwodotZero schema and is a generic way to refer to a message sent to an IdentityProvider to ask it to authenticate a user and issue a SAML assertion.
In SAMLTwodotZero, this is actually done using an XML message called an AuthnRequest .
In Shibboleth now, a set of query string parameters passed on a URL is used as a simple, unsigned AuthnRequest.