POP stands for Post Office Protocol and is used by mail servers and email programs to talk to each other accros the Internet, and allow users to download their email. In essence a POP Mailbox describes the protocol that allows you to retrieve email and the physical location that emails are stored in on the mail server.
A POP3 Mailbox cannnot be set up with the same address as and existing Forwarder and vice versa.
When your hosting account is initially set-up a default catchall POP3 Mailbox is created. Anything that is emailed to your domain name e.g. yourname@yourdomain.com will end up in this default Mailbox.
If the hosting is being used by more than one person i.e. a business or organisation, it is quite common that one Mailbox which catches all emails sent to the domain name may not be appropriate. It may be necessary for individuals to have their own Mailboxes so that they can retrieve email independently of other users and onto their own computers.