Setting up a mail server using a freenom domain

I’m trying to follow this tutorial on setting up a web server. I’m a complete newbie to this stuff, so bear with me. I created a domain with freenom, and added Digital Ocean’s nameservers to it. What has that accomplished? If I go to the website, nothing happens. I’ve experimented with django and deployment in the past, but the website would only display something when running manage.py’s runserver function? How does it run without doing so manually like this?

That tangent aside, I’m focusing on spinning up a mail server. Say my domain name is example.tk. I need to set up MX records, so I went to MX records in the DNS record of my droplet. I decided mail.example.tk would be a nice subdomain name, so I put that in the hostname box. What do I put for “MAIL PROVIDERS MAIL SERVER”, though?