SSL = Secure Socket layer
POP = Post Office Protocol
IMAP = Internet Message Access Protocol
SMTP = Simple Mail Transport Protocol
What are ports?
Ports are not physical, they are more like a concept. They are like a label or designation. Think about it like this: Let's say that a friend says that they will leave a message for you at their apartment building. There are 20 apartments adn there are 20 hooks at the entryway where you can hang messages. The friend lives in apartment 12. He tells you that he will leave the message for you on hook 8. You ask why the message will not be left on hook 12. The friend says, "Because I am going to leave it on hook 8. If pull a message from any other hook, it will be the wrong one." So, essentially the two parties have decided that hook 8 will be designated as their point of communication. It is arbitrary. There is no rhyme or reason to it. It just is. So, let's say your email app will be setup to "listen" for messages at port 995. The app will not accept messages designated for any other "port". Let's say that an email server for outgoing messages is "listening" at port 465. If you email app sends messges to any other port, the email server will ignore them and the messages will never be sent. Hope that the explanation makes sense. If I said something that wasn't quite correct, feel free to correct me.
An important thing to remember about Ports.
They often vary across email providers.
While there are some 'default' and 'suggested' port numbers to use, often Email Providers will use whatever they choose.
So, when setting up email accounts manually, always try to find settings for your exact email provider. Usually, the home page of the email provider will have a link to settings.