Like renting a street address on the web, a domain is the name people type to reach your site or API.
You buy a domain from a registrar, the company that sells names like myapp.com. Owning it lets you control where the name points and who can send email from it.
It matters because users need a trustworthy place to find you and because email providers look for verified sending domains. The Astro marketing site lives at the domain on Vercel, and Resend needs the domain verified so your emails land.
In practice: pick a name, pay a yearly fee, then set DNS so the name goes to your site. You can also create subdomains, names like api.myapp.com, for different parts of your system.