As far as I can tell, you pay $99 or whatever, and it periodically sends a text to your iPhone telling you it hasn't actually backed up anything in XX weeks.
Kind of a weird business model but then they didn't ask me.
* You need a Mac or iOS device to join, though. And you can pay for additional storage if you want ($20, $40, or $100 per year), but that's not necessary.
And then next year to keep iCloud "free", you need to buy more overpriced goods from Apple.
Must have been a pretty recent change though.