IMAP stands for the Internet Mail Access Protocol. Like POP-3, it allows e-mail reader programs to retrieve their e-mail from the server. Unlike POP-3, IMAP also provides support for keeping your mail in several folders on the server, so you can organize it more easily.
If you want to allow e-mail readers to access their mail with IMAP, you should enable the IMAP service on your NetWinder. In that case, you will usually want to enable the POP-3 service as well.