The internet's phone book
DNS (Domain Name System) turns your easy-to-remember domain into the number (IP address) computers use. Without it you'd have to memorise numbers instead of names.
You could, but numbers are impossible to remember. DNS lets people use a name while computers quietly use the number behind it.
Nameservers point the way
Your domain's nameservers decide which DNS records are in charge. Point them at your host and your site loads; point them wrong and nothing works — even though the domain is fine.
Records do the jobs
An A record points your domain at a server, an MX record routes your email, and a TXT record holds verification (like SPF/DMARC). Small entries, big effects.