OK.
The last link from you was helpful!

But one last question to see if I got this right:

Imagine following scenario:

The proxy_buffers are empty. A client wants to visit let's say google.com. Our php script gets the content of google.com, sends it to the client and nginx will also buffer the response from google.com in a proxy_buffer.

Is that right so far?

So the next time if a client wants to visit google.com nginx will take the response from a proxy_buffer without making a request to google.com.

Right?

If I understood it right, proxy_buffering is caching?

The term "proxied server" confused me...

Thanks Alexandr Gomoliako!