To my understanding, metadata exchange (in WCF) means whether the service will offer a WSDL or other types of metadata to a client to enable them to autogenerate a proxy for the web services.
By default it is not enabled to exchange metadata, so its something that has to be added as an extra endpoint.
The endpoint works as any other endpoint (see link), where the contract type IMetadataExchange is mandatory. This contract type is a predefined service contract which is found in System.ServiceModel.Description namespace.
The endpoint can have different kinds of bindings like MexHttpBinding (meaning HTTP), MexHttpsBinding (HTTPS)
From a web service perspective it seems a bit strange that you would have to specifically add an endpoint to allow metadata transfer. But since WCF is a general communication framework I'm sure there is lots of places where its better not to have a service sending out metadata.