POP3
In plain terms
An old email protocol that downloads messages to one device.
Going deeper
POP3 retrieves email from a server by downloading it (often deleting the server copy), so mail lives on a single device. It predates multi-device sync needs.
Under the hood
Unlike IMAP, it doesn't sync state across devices; largely superseded by IMAP and webmail.
Examples
Legacy email clients, single-device mail, simple downloads, IMAP alternative.
Think of it like...
Like taking all your mail home from the post office, leaving nothing behind to read elsewhere.