Post Office Protocol, commonly known as POP, is a standard protocol used by email clients to retrieve emails from a server. It allows users to download their emails to their local devices, making it easier to access messages without needing a constant internet connection. The most widely used version is POP3, which supports various features like message deletion and multiple email accounts. When a user connects to their email server using POP3, the protocol transfers emails from the server to the client. Once downloaded, the emails can be read offline. However, this means that emails are typically removed from the server, which may limit access from other devices.