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 stores the current server configuration, modify it here

sites-enabled is the server configuration that is activated and used (create a shortcut from the files of sites_available to sites-enabled)