(via wiki.nginx.org)
The stub_status module provides the ability to get some stats from nginx...
active connections -- number of all open connections including connections to backends
server accepts handled requests -- nginx accepted connections, handled connections (no one was closed just it was accepted), and handles requests (requests per connection = handles requests / handled connections)
reading -- nginx reads request header
writing -- nginx reads request body, processes request, or writes response to a client
waiting -- keep-alive connections, actually it is (active - reading + writing)
