nginx is an open-source HTTP server.
Let’s say you’re running a server with nginx and you have two domains domain1.com and domain2.com. You also have the HTML of two websites sitting at ~/web/domain1.com/ and ~/web/domain2.com/. And now, of course, you want visitors that go to domain1.com to be directed to the website at ~/web/domain1.com/ and in an analog way for the other domain.

For this you make the sites available by creating their configuration files

/etc/nginx/sites-available/domain1.de

server {
	listen 80;
	listen [::]:80;

	root /home/YOUR_USERNAME/web/domain1.de/;
	index index.html;

	server_name domain1.de www.domain1.de;

	location / {
		try_files $uri $uri/ =404;
	}
}

Substitute all domain1.de with domain2.de and that’s your /etc/nginx/sites-available/domain2.de.

These sites are now available, but not enabled yet. The idea is to configure everything in /etc/nginx/sites-available/<file>, and to then have symbolic links in /etc/nginx/sites-enabled/ that point to the configuration files you want enabled.

So enable the sites:
sudo ln -s /etc/nginx/sites-available/domain1.de /etc/nginx/sites-enabled/domain1.de

sudo ln -s /etc/nginx/sites-available/domain2.de /etc/nginx/sites-enabled/domain2.de

Make sure your symbolic links work by issuing ls -l /etc/nginx/sites-enabled/ which should show

total 0
lrwxrwxrwx 1 root root 44 Mar 23 19:10 domain1.de -> /etc/nginx/sites-available/domain1.de
lrwxrwxrwx 1 root root 41 Mar 23 19:15 domain2.de -> /etc/nginx/sites-available/domain2.de

Assuming Ubuntu, restart nginx: sudo service nginx restart.

Alright, you’re good to go!