It's actually fairly straightforward.
Domino Everyplace allows you to access your Domino server (and out-of-the-box only mailfiles) via the micro-browser found on most cell phones and PDA devices.
Once you install it, it loads when the server loads, and you access it via pointing the browser on the handheld device to www.yourmailserver.com/servelt/deas
It will ask for a name and a password (which line up conveniently with your 'normal' internet name and password) and it'll take you to your mail, calendar, to-dos, etc.!
Alan has pointed you at the official documentation in his response ...