I've heard this phrase used several times, and the first few times I was mystified. What is this magical 'executive summary' feature in Outlook? Is it some magical content-parser that extrapolates the key terms from a message? Can it slice, dice and put a home cooked meal on the table by the time father gets home?
Eventually, I uncovered the truth - this is just another way of referring to AutoPreview (or 'three line preview' as it is also called). The reason it's called the executive summary feature by some is a way of emphasizing how to structure email to make sure that upper management actually reads it - put the most important information in the first three lines of the email.
But really, this is good advice for almost any (work-related) email - make sure the most important point you want to make (or a summary of the most important points, or your 'asks', etc) is at the very top of the mail. Make your point loud and clear - as the recipients are skimming the email, they shouldn't have to read through paragraphs to find out why you're sending the mail.