Earlier quoted context omitted.
Free certificates last a week. You can also pay Apple $99 to get a certificate that lasts a year, or pay a "UDID registration" website around $10 for the same (with some limitations).
What’s UDID registration website?
>Starting at $8.99, we also provide Certificate & Provisioning files that can be used to install any app, including apps you've created and those not found on the App Store.
Looks like they're essentially "sharing" developer accounts.