NS, NameServer are a necessary technical component of the DomainName system. They hold the domain names like "google.com" and translate these names to the numeric IP-addresses (e. g. 184.108.40.206) of the computers where the domain names are located.
These translations happen all the time, so the NameServer must always be actively working. Because this can hardly be guaranteed, one must always have two NameServers in parallel. During domain registration you have to give the names of these two NameServers. Typically you do not run your own NameServers, but use those of your provider.