If you are coming from Apache, the "sites-available" and "sites-enabled"
directories will be familiar.
These directories are used to define configurations for your websites.
Files are generally created in the "sites-available" directory, and then
symbolically linked to the "sites-enabled" directory when they are ready
to go live.
...
In the "nginx.conf" file, we can see that the end of the "http" block has:
include /etc/nginx/conf.d/*.conf;
include /etc/nginx/sites-enabled/*;
How To Configure Nginx
sites-available是存放目前的server配置, 在這裡修改
sites-enabled是啟動並使用的server配置（從sites_available的檔案建立捷徑到sites-enabled）