What is a subframe?

Question

Google Chrome

Hello.
Google Chrome has a Task Manager (Shift-Esc).
If you open it, some subframes are sometimes visible there.
Image:

Can you please explain what it is?
It doesn't look like a separate tab (Tab).
Is it some part of a web page?
How does this relate to the real world (html? extension? connection from the page? wtf??)
In general, I don’t understand anything!

Answer 1

This is a site isolation feature. Google did it kind of to protect against spectre. Each new site launched is isolated.