POP3 stands for Post Office Protocol version 3 and refers to the protocol used to receive emails to your local email client from a remote server.
POP3 downloads mail from the server and often deletes on the server after it has done so. This means that depending no when your mail client checks for mail, you will only receive the email on one device or mail client. This also means that when you delete an email from one device, it will not be deleted from another. For example, if you receive emails on your smartphone, deleting them won’t also delete them on your desktop mail client, so you will have an email that you have already viewed appear as unread.
Since POP3 downloads emails from the server and saves them in a local storage area, losing or damaging your local storage may cause you to lose all your emails.