The Post Office Protocol 3 (POP3 for short) is a method for receiving emails.
It is a transfer protocol through which an email client moves emails from the mail server to your device, and these emails are deleted from the server in the process. If you only receive and read your emails on one specific device, you can use POP3 as a means of making sure your email storage space does not get filled.
However, it is recommended that you use the IMAP Protocol instead. Using IMAP allows viewing all emails on multiple devices, as well as helping to prevent accidental loss of your messages by keeping them centrally stored on the email server.