SAML metadata file is an XML document that contains all the information necessary for the interaction between the SAML-enabled identity and the service provider. The document contains  URLs of endpoints, information about supported bindings, identifiers, and public keys.

It also contains an X.509 certificate (a standard defining the format of public-key certificates). 

An example SAML metadata file looks like this:

When the Identity provider sends an assertion about the user to the ACS URL, the private key signature in the assertion is checked against the public key present in the metadata file, before authorizing the user to access the service.