In simple terms - it basically identifies the audience [Service providers] to whom this assertion is intended for.
Typically, you will identify your service in your identity provider by a name [for eg, https://sp.example.com/SAML2], IDP can add audience element with this identifier. So in this case, if you are validating at Apigee - you would, typically, have an id associated for Apigee [lets say http://sp.apigee.net/saml], you can check if your ID is present in the Audience element.