What is "mexHttpBinding"?

It is a binding that returns metadata so you can build a proxy at the client side. See here. More here as well.

mexHTTPBinding allows to use WS-MetadataExchange over HTTP. You don't need it unless you implemented policies which may change over time and you need to discover your service with UDDI for example

Many people says it's needed for creating client side proxies. This is not true. For client side proxies you need to publish WSDL. You can do that by specifying in your service behavior.

Shortly, you don't know it -> you don't need it.

mexHttpBinding is used while adding a new Service Reference in a Project in Visual Studio 2013 (DEBUG) that will consume your WebService (providing the mexHttpBinding). After you've added the Service Reference successfully you might disable the mexHttpBinding.

It is all described in the App.config when you're creating a WCF Service Library Project in the Visual Studio 2013 ( Solution >> Add New Project >> Visual C# >> WCF >> WCF Service Library)