IMAP (Internet Message Access Protocol) is a client-server approach to email in which email is kept on the server. When you work with email, your email program (the client) connects to the post office server to access your email. Since your email stays on the server, you can easily access it from multiple workstations and programs just like you can access your voice mail messages from any phone.
IMAP is one a legacy email system previously offered by IS&T and an alternate protocol you can use to contact the MIT Exchange servers. The other is Exchange.