What exactly show nginxplus_upstream_server_response_time metric?

[DANic-git]

In the description:
Average time to get the full response from the server

What does "average" mean here? Average of what?

When I visualize this metric on a time series graph, I see something like:

sum (nginxplus_upstream_server_response_time{
  upstream="default"
}) by (pod)

What does this show?