DNS/nameserver/DNS Server
A website is located on a server identified by an IP address. The IP address works well for computers but it is difficult to remember for humans: that is why the Domain Name System (DNS) protocol was developed.
Technically speaking, the DNS is in fact a database that associates domain names with IP addresses. This is why the DNS is often compared to a telephone directory that associates persons names (equivalent to domains) with telephone numbers (equivalent to IP addresses).
If web users want to go to a given website, they will not type in the IP address, as it is too complex. Instead, they will type in their browser the domain name that has been assigned to the website. This starts a process of decoding the website the user is looking for and routing his traffic to the right web server. The nameserver, acting in the middle of this process, holds all of the records for which websites are located and where, as it is able to translate the domain name into an IP address.
The DNS makes it possible, in particular, to reach the web server of a specific domain or a mail server to which a user wishes to send an e-mail.
AFNIC has published a short video explaining the Domain Name System (DNS) very well in French: https://www.youtube.com/watch?v=dcIrB8qRCbA&feature=youtu.be.