DNS
Like a phone book for the internet, DNS turns a name you buy into the numeric address of the computer that serves your site.
When someone types your name, their device asks DNS where to go. DNS replies with an address so the browser or app can reach the right computer.
This comes up when you point your name to Vercel for the Astro marketing site, or when you verify a domain for Resend to send email. You edit DNS at your registrar, the company where you bought the name.
In practice: you add DNS records, small instruction lines. An A record points a name to a number address, and a CNAME points a name to another name. Changes can take minutes or hours to spread worldwide.