A summary is a buffer containing one line per message to give you an overview of the mail in an Rmail file. Each line shows the message number and date, the sender, the line count, the labels, and the subject. Moving point in the summary buffer selects messages as you move to their summary lines. Almost all Rmail commands are valid in the summary buffer also; when used there, they apply to the message described by the current line of the summary.
A summary buffer applies to a single Rmail file only; if you are editing multiple Rmail files, each one can have its own summary buffer. The summary buffer name is made by appending ‘-summary’ to the Rmail buffer's name. Normally only one summary buffer is displayed at a time.