what is the function about <iframe> element?

Answer 1

<iframe> is a container to handle complete nested html document, usualy remotly, but not always... You can dinamically write inside also ;)

Answer 2

<iframe> is to display remote content on your webpage.
Like here:
<iframe src="https://www.yahoo.com">
<p>Your browser does not support iframes.</p>
</iframe>

Answer 3

thank's for helping me.