|This page in a nutshell: When editing, be sure to summarize your contributions.|
An edit summary is a brief explanation of an edit to a Wikipedia page. Summaries help other editors by (a) providing a reason for the edit, (b) saving the time to open up the edit to find out what it's all about, and (c) providing information about the edit on diff pages and lists of changes (such as page histories and watchlists).
According to the consensus policy: All edits should be explained (unless the reason for them is obvious)—either by clear edit summaries, or by discussion on the associated talk page. To add a summary, type in the text entry field in the Edit summary box located near the bottom of the Editing page. The box looks like this: