POP3, the Post Office Protocol version 3, is one of the oldest standards for retrieving email. Its design is simple: a client connects to the server, downloads new messages to the local device, and by default deletes them from the server afterward. The mailbox effectively lives on the device that pulled the mail.
This download-and-delete model made sense in an era of a single desktop computer and expensive server storage, but it fits poorly with today's multi-device habits. If POP3 removes messages from the server, a message downloaded to your laptop will not appear on your phone, and read status is never synchronized between devices.
Most POP3 clients offer a leave a copy on server option to mitigate this, but even then there is no synchronization of folders or flags - each device maintains its own independent view. For anyone using more than one device, IMAP is the far better choice because it keeps everything in sync on the server.
POP3 still has niche uses: archiving mail locally, minimizing server storage, or working in low-bandwidth environments where downloading once and reading offline is preferable. It typically runs on port 995 over TLS. Like IMAP, POP3 only retrieves mail; sending is always handled by SMTP.