what does the "main" pointor in request struct mean?

[4everpan]

I found in Nginx that:
struct ngx_http_request_s {
    ...
    ngx_http_request_t               *main;
    ...
};
what does this argument mean?

I also found a similar argument in Apache:
request_rec* request_rec::main  
//Pointer to the main request if this is a sub-request (see http_request.h) 

so,
Do they have the same meaning?
and if yes, what dose a main request and a sub-request mean?  What's the difference between the 2 http requests?

hope anybody's help. Thx~~

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,90805,90805#msg-90805