Ports are basically lines of communication for programs over the internet. Some examples are regular internet traffic which uses port 80, FTP traffic which uses port 40, and steam which uses several ranges of ports.
Ports are used to pass information back and forth between computers/servers on the internet.
Opening ports means setting up your router to allow specific groups of ports to be directed to a machine on the internal network. If the server is being hosted out of his home, your friend will have to do this before anyone not on his home network can do anything.
If the server is hosted by an online site, you don't need to worry about any of that!