A Jabber ID is an account identifier similar to an email address. It is what you will log into your app with and how you will send and receive text messages with your new phone number. You can use a Jabber ID from any standards-compliant service with JMP. Most apps will walk you through setting up an account after you install them, then come back here to continue.
Contact support to get set up with a Snikket instance on your own domain or a free subdomain. This will also get you the app and a Jabber ID, after which you can proceed with getting your phone number.
You don't need this web-based setup at all, you can get the Cheogram app and when prompted select JMP, then search for a phone number in the app.
Any standards-compliant Jabber app on any platform will work.