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).
For example, if web users want to go to PlanetHoster’s website, they will not type in the IP address, as it is too complex. Instead, they will type in the domain name that has been assigned to the website, namely planethoster.com in our exemple. The DNS server will convert the domain name entered by the user into an IP address that will allow to obtain the site data.
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. You can watch it here in French.