Nameserver NS record

The servers that give the authoritative answer about your domain. Everything in DNS sits there: the A record of the website, the MX record for the mail, SPF and all the rest. Whoever runs the nameservers manages the records.

Which nameservers apply to a domain is set at the registry, for .com domains through the registrar at Verisign. That is the only switch thrown there. Everything else happens at whoever operates the nameservers.

Two is the usual minimum, so the domain stays reachable when one of them goes down.