It's a distribution completely centered around the Nix package manager. This basically allows you to program how your system should look using one programming language. If you want an identical system, just copy that file and you're set.
Instead of installing packages through a package manager one at a time and configuring your system by digging into individual config files, NixOS has you write a single config file with all your settings and programs declared. This lets you more easily configure your system and have a completely reproducible system by just copying your nix files to another nixos machine and rebuilding.
It’s also an immutable distribution, so the base system files are only modified when rebuilding the whole system from your config, but during runtime it’s read only for security and stability.