Server blocks are the actual configuration pieces that tell the server what to serve and on which port to listen. We can define multiple server blocks in the sites-available folder. On Ubuntu, the location will be:

/etc/nginx/sites-available

On macOS X, the location will be:

/usr/local/etc/nginx/sites-avaiable

Until we copy the sites-available to the sites-enabled directory, the configuration has no effect. So, always create a soft link for sites-available to sites-enabled for every new configuration you create.