Nameservers, "name servers", or "DNS servers" are special servers that are configured to answer requests to translate a domain name into an IP address.
The public domain name database allows computers to determine the nameservers for any given domain name. Nameservers are necessary for all domain names, since traffic on the Internet is actually routed to its destination by IP address, not domain name. Before any computer can connect to your site via domain name, it needs to look up the IP address.
A domain name will generally specify a primary nameserver (the one that should be tried first) and at least one secondary nameserver to be used if the primary is unreachable.