What's the main concept about Iframe in HTML ?

Answer 1

The <iframe> tag specifies an inline frame. An inline frame is used to embed another document within the current HTML document.
The <iframe> tag defines a rectangular region within the document in which the browser can display a separate document, including scrollbars and borders. ... The src attribute is used to specify the URL of the document that occupies the inline frame.
https://www.tutorialspoint.com/html/html_iframes.htm

Answer 2

Website inside website is the main concept.