Web Server Link Domain

I have a web server and I want to link it to a domain. Will making an A Record pointing to the ip of the server link the domain? Also what is the "main port" for requests. By main port I mean the port you go to when you make a request. My site is using Flask and I have to specify the host and port to run it on. So what is the "Main host" and "Main port". I think the main host is and port is 80, 443 or 53.