Unraid is a proprietary operating system, Unraid is developed and sold by Lime Technology Inc. To use Unraid you need a license and this costs money. It starts at 59$. One time, so you don’t sign up for a subscription.
Unraid Free of charge
If you want to have a look at Unraid first, you can easily use the free 30-day trial version of Unraid. Only if Unraid convinces you, you buy a license. How to install Unraid you can read here.
Unraid is one of the best operating systems for home servers and home NAS systems. How Unraid compares to the probably best known competitor FreeNAS or TrueNAS you can read here.