It's a version of Linux. NixOS lets you do a ton of configuration from a text file, including which software should be automatically installed and, for the most popular software, how it should be configured. It's complicated and there's more to it, so it's not for everyone, but it's really useful if you manage servers or if you're worried about losing/replacing your hard drive because you can get really close to a fully set up computer REALLY quickly and easily with that config file once you have it.