POP3 stands for Post Office Protocol-version 3 and is a server which stores your email. When an email is sent to you, it is stored on a server (called the POP3 server) until it is downloaded via your email client (Outlook, Netscape Mail, etc.).
When you purchase a hosting package with 123-reg you will automatically be given a number of mailboxes. These mailboxes can be assigned to a specific email address, allowing you to access them using a program such as Outlook Express or Mozilla ThunderBird.
For more information on the POP3 protocol check out this helpful knowledge video
Please note: The message will be removed from the server unless you tell the mail client to “leave on server”.