POP stands for Post Office Protocol, and was designed as a simple way to access a remote email server. The most recent version is POP 3, and is supported by virtually all email clients and servers.
POP works by downloading your emails from your provider's mail server, and then marking them for deletion there. This means you can only ever read those email messages in that email client, on that computer. You will not be able to access any previously downloaded emails from any other device, or with any other email client, or through webmail.
IMAP stands for Internet Message Access Protocol, and was designed specifically to eliminate the limitations of POP.
IMAP allows you to access your emails from any client (Apple Mail, Outlook, etc.), and any device (mobile, tablet, desktop), and webmail login at any time, until you delete them. Your devices seem to “communicate” with each other and you are always seeing the same emails, no matter how you access your provider's server.
Since your email is stored on the provider's server and not locally, you may run into email storage limits, when using IMAP.