Like a reliable mailroom for your app, it sends emails such as sign-in links, receipts, and alerts through an easy web service.
Imagine handing letters to a trusted mail desk that gets them delivered and tells you what happened. Resend is an email delivery service that sends transactional emails like sign‑in links and receipts for your app.
It matters because running email yourself is fragile: spam filters, server setup, and deliverability are tricky. A service gives you an API, which is a set of rules your app uses to talk to it, and handles the hard parts.
In practice you verify a domain by adding small records at your domain host to prove ownership, create templates for common messages, store an API key, which is a secret pass for your account, on the server, and call the service to send emails.