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~~