IMAP is an extension to POP3, the usual method of collecting e-mail. It allows you to read your emails from any computer, not just the computer where you have set up your email program.
You do not have to use it, and your mail will work fine if you just use POP3 with the default settings.
If you do wish to use IMAP, all you have to do is create a new account and, when you are prompted for the e-mail server name, enter either imap.ntlworld.com (if your email address ends in @ntlworld), or imap4.blueyonder.co.uk (if your email address ends in @blueyonder.co.uk). Make sure that you specify that the Incoming mail server is an IMAP server and not a POP3 server.
Don't forget that you still need to have smtp.ntlworld.com or smtp.blueyonder.co.uk configured as your outgoing [SMTP] server!
