IMAP is a e-mail retrieval protocol that allows you to keep your e-mail on the server. This has a major advantage over POP3 in that you can check the same e-mail messages from anywhere that supports IMAP. It is now pretty ubiquitous and considered the standard e-mail checking protocol to use. Suso has supported IMAP since around 1999 and was one early adopters of using it in web hosting.
Most likely you use an email client that already supports IMAP, so all you have to do in order to use it is setup your e-mail account and choose IMAP for the incoming mail protocol.