A nameserver is a type of DNS server. It is the server that stores all DNS records for a domain, including A records, MX records, or CNAME records.
When you add custom nameservers to your .realtor or .realestate domain, the DNS records are managed by whoever manages those servers.
Almost all domains rely on multiple nameservers to increase reliability: if one nameserver goes down or is unavailable, DNS queries can go to another one. Typically there is one primary nameserver and several secondary nameservers, which store exact copies of the DNS records in the primary server.
Here's an example of custom nameservers.