opening and closing ports

An open port simply means that some program is running and listening for input on that port. The program may or may not respond.

A closed port means that no program is listening on that port.

So to "open" a port, you have to start a program that will listen on that port. To close it again, stop that program.

A "stealth" port is a port that does not respond to "knocks" from the Internet (pings, I guess), whereas a closed port might respond to a knock saying "I'm here but closed" even though there is no program actively using the port. That would let a hacker know that there was a computer at that port.