Any server machine makes its services available to the Internet using numbered ports, one for each service that is available on the server. For example, if a server machine is running a Web server and a FTP server, the Web server would typically be available on port 80, and the FTP server would be available on port 21. Clients connect to a service at a specific IP address and on a specific port.
Note that there is nothing that forces, for example, a Web server to be on port 80. You could put your web server on port 998, someone on the internet could connect to your server with the URL http://xxx.yyy.com:918