Edit summaries are a way of communicating with other contributors via the "Summary" box available when you save each edit you make.
By leaving a brief but specific description, it gives others the chance (via their watchlist, recent changes, or the page history) to ascertain quickly whether the edit might be something they want to follow up on. For instance, the edit summary "formatting" may be of interest to one contributor, whereas a different person might have something to offer on an edit summarized as "revising treatment description in light of new findings".
Frequently saving your work will allow you to make your edit summary more specific and helpful to others.
Note: Most source editor code also works in the edit summary bar, so if you like, you may include a link to an MEpedia entry or policy, or ping another editor. (For more information on using Source Editor, please see Help:Source editor.)