IMAP means Internet Message Access Protocol and POP3 means Post Office Protocol
POP3 downloads email from a server to your device and after downloading it gets deleted from server. So POP3 is useful when you want to keep all your email only on one computer for some security reasons.
IMAP is the better option when you need to check your emails from multiple devices say laptop and a smartphone as after the download mail is not deleted you just sync your mail client with mail server.