Internal apps are company-specific apps developed by your organization that you may not necessarily want to be searchable in the public app store, but you want your users to have access to this application from their device. You will still add these apps to Google Play for Work so that they are available to Android for Work specific users.
You will upload your application by logging into the Google Play Developer Console with your enterprise credentials. There is an additional option to enable, Restrict Distribution, which only allows users of your domain to view this application on Google Play for Work (the badged play store). Once you've added your internal application to the developer console, these apps will be treated as public applications. You will follow all directions in this section to get these apps approved, uploaded and assigned to your users.