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.). The email is then downloaded on your server into your mail client or onto your mobile device for you to read and respond to.
This is different to the IMAP email protocol, which stored the email on the server, and allows you to view it through your client, rather than by downloading it directly.
For more information on the POP3 protocol check out this helpful knowledge video
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.
Please note: The message will be removed from the server unless you tell the mail client to “leave on server”.