It doesn’t allow remote access per se — You need to be within (good) WiFi range of the local area network to which your Roon core is connected.
Within those parameters, it’s still very useful, since you can listen on an iOS device as you move around your area, and you can create all kinds of ad hoc Roon zones by connecting your iDevice, either wirelessly (e.g., by Bluetooth) or via cable to a wide range of audio systems.
If that’s what you’re looking for, you just need to install iPeng 9.3 on your iDevice and do the in-app purchase of the Playback module. Once that’s set up and iPeng is running on your iDevice — you already said you have Roon’s Squeezebox support enabled — Roon should see your iDevice as an available Squeezebox endpoint, and then you need to configure it as you would any other Roon zone. That’s pretty much it for the basics, but you should read through the last 24 hours worth of posts in this thread for some very useful tips.