A summary is a buffer containing one line per message to give you an overview of the mail in an IMAIL folder. Each line shows the message number, the sender, the flags, and the subject. Almost all IMAIL commands are valid in the summary buffer also; these apply to the message described by the current line of the summary. Moving point in the summary buffer selects messages as you move to their summary lines.
A summary buffer applies to a single IMAIL folder only; if you are editing multiple IMAIL folders, each one can have its own summary buffer. The summary buffer name is made by appending `-summary' to the IMAIL buffer's name. Normally only one summary buffer is displayed at a time.