The Document is like the top level HTML structure of your Next.js application. By default it looks like this:
import { Html, Head, Main, NextScript } from 'next/document'
export default function Document() {
return (
<Html>
<Head />
<body>
<Main />
<NextScript />
</body>
</Html>
)
}
As you can see it looks pretty close to an HTML document but with custom components instead of the native tags. Here, you can introduce custom behavior like fonts, scripts, icons, etc.