Email is confusing. There are multiple protocols and so many possible combinations of devices and settings it can be tricky to get it working sometimes.
One thing many people don't understand is the difference between POP3 and IMAP protocols.
Firstly, it is important to understand that they are both incoming email protocols (outgoing is called SMTP).
POP is dumb. It collects emails and then either deletes them from the server or leaves them there - this is a setting in your mail client (outlook, mac mail, thunderbird etc). If emails are deleted then they aren't available for anyone else. There is nothing else POP does.
IMAP has some smarts. It syncs between the device(s) and the server. All emails are kept on the server and also downloaded to each device. If you read on one device it is marked read on others. Delete on one and all are deleted, same with writing etc. It is the best protocol if multiple devices are used.
For most people, IMAP is the way to go. You are setting yourself up for the future and there is nothing lost if you don't use all the functionality. However with POP it is hard to switch to IMAP and if you have chosen to delete emails from the server and your computer dies, your emails are all lost (cue ominous music).