[Answered ]-Nginx : meaning of "expires 30d"

This adds two HTTP headers to the responses (Expires and Cache-Control). Those headers are used by the browsers to cache content, avoid doing the same requests for static content each time a page is loaded.

expires 30d means that all content in static and media folders will be cached by browsers during 30 days, but nothing will be deleted and you won’t need to regenerate anything in the server.

expires max set the Expires header to the value “Thu, 31 Dec 2037 23:55:55 GMT”, and the Cache-Control to 10 years.

See the nginx documentation for more details: http://nginx.org/en/docs/http/ngx_http_headers_module.html

For more info about HTTP caching see http://www.mobify.com/blog/beginners-guide-to-http-cache-headers/