DNS
DNS is the phone book of the internet: it turns a name like yourshop.com into the place where your site lives.
In a simple example
You buy a domain, then tell the DNS where your hosting is. After that, the name opens your site.
Why it matters to you
If the DNS settings are wrong, your site does not open, even when the site itself is fine.
Do not confuse with
DNS is not hosting. It only points the name to the hosting.