ng is a common shortcut for "Next Generation" (ex: syslog-ng…)
Unless in Japan where it means "no good" hence :NG emoji:
O-oh, thank you! :)
For me it means 'not gonna'.
Or Star Trek TNG.
It's supposedly meant as an upgrade to nghttp2 which brought HTTP/2 and is used by eg Apache httpd, though I have no idea it's by the same authors, or has API backward-compat, etc. At a glance the linked doc doesn't answer these and other questions either such as a license terms and/or git link. Btw the Read The Docs menu seems broken on mobile (displays a fold-out menu with details when that should probably be hidden).