Protecting Nginx against DDoS attacks by Fail2Ban
 

Hello,
To protect Nginx against DDoS attacks by Fail2Ban I found the following tutorial:
https://easyengine.io/tutorials/nginx/fail2ban/

What does HOST mean in the following line? Should I enter the IP address of the host or the domain name of the website?
Thank you.

It's the "placeholder" in the regex that tells F2B where to extract the IP address from.
None of the above. Try reading some documentation such as https://fail2ban.readthedocs.io/en/latest/filters.html rather than blindly relying on "How To" guides.