IMAP lets you access email stored on that server. So imagine that you are on a network at work, the emails you receive is not stored on your computer but on a specific mail-server. So every time you look in your inbox your email-client (like outlook) fetches the emails from the mail-server using imap.

IMAP is a lot like pop3. But with IMAP you can access your email from various devices. With pop3 you can only access them from one device.

Port 993 is the secure port for IMAP