Actually, IMAP and POP are the protocols using which you can download messages from mail servers on your computer and access them with the help of mail clients such as Microsoft Outlook, Mozilla Thunderbird etc. The main advantage of these protocols is that user can access his/her emails via mail client.
IMAP and POP are two different protocols. There are many differences between these two. It is that IMAP (Internet Messaged Access Protocol) always syncs with mail server so that any changes you make in your mail client will immediately appear on your webmail inbox.
In POP (Post Office Protocol), your mail client account and mail server are not synced. It means whatever changes you make to your email account in the mail client will not be transmitting to the webmail inbox.
Activating the protocols, for example take the Gmail you can find options to activate both the protocols: POP and IMAP (Go to settings -> Forwarding and POP/IMAP).