The .rar extension indicates a compressed archive created with WinRAR. The "NS" prefix typically refers to one of three things in the gaming world:
Right-click the file and select "Extract Here" to see the contents. Where to Find it Today NS.rar
This is the most common origin. Older forum posts from Unknown Worlds refer to "ns.rar" as a distribution package for Linux server installs or specific beta patches. NS.rar