A Nameserver (sometimes called a DNS server) is what translates a domain name or computer hostname to an IP address, which is a numeric pointer that identifies a computer on the Internet.
Each computer / server /etc has an IP address that looks like four numbers separated by a dot
1.2.3.4
192.168.0.2
10.1.2.3
etc
Instead of needing to remember all those numbers for your favorite websites, we use the DNS (domain name system) to translate domain names into IP addresses. The Nameserver is the part of the DNS system that translates all that information.
When your computer wants to know how to access www.google.com, it sends a query to your ISP's DNS servers. Those servers are responsible for answering your computer's questions. If your ISP's servers don't know the answer, then they have a process where they ask the central Nameservers for the Internet, who then point them to Google's Nameservers for the answer. Since Google's Nameservers are authoritative for google.com, www.google.com, gmail.com, etc, your ISP's servers ask Google's servers for the correct IP address, and then tell your computer so it can ask the right server for the web page. Your ISP then caches that information so it doesn't have to ask again for a while, in case another computer asks the ISP for that information.
The Nameservers that A Small Orange uses for the domains it hosts are: