POP3 is an acronym for Post Office Protocol version 3.
POP3 is the primary communications protocol or language for receiving email from an email server or provider. POP3 is used by email programs like Thunderbird, Microsoft Office Outlook, and others use to communicate to mail servers when downloading your email. It can also be supported by stand-alone email programs on other devices.
When POP3 is used, email messages are moved to the computer to which they’re being downloaded – meaning that after being successfully downloaded, they are removed from the email server on which they were stored.
POP3 is also sometimes used to transfer email from one email provider to another. For example, accessing your Outlook.com email using Gmail can be performed by configuring Gmail to use POP3 to fetch and transfer the Outlook.com email to your Gmail account.