The NS, or Name Server records of a domain, reveal which servers manage the Domain Name System (DNS) records for it. Setting the name servers of a given hosting provider for your domain address is the most convenient way to point it to their system and all its sub-records are going to be managed on their end. This includes A (the IP address of the server/website), MX (mail server), TXT (free text), SRV (services), CNAME (forwarding), and so forth, so if you would like to modify any of these records, you're going to be able to do it using their system. Put simply, the NS records of a domain reveal the DNS servers that are authoritative for it, so when you try to open a web address, the DNS servers are contacted to obtain the DNS records of the domain you want to access. That way the website that you'll see will be retrieved from the correct location. The name servers normally have a prefix “ns” or “dns” and every single domain address has at least 2 NS records. There isn't any functional difference between the two prefixes, so which one a website hosting provider will use depends completely on their preference.