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. When you edit a page, there is a small text entry field labeled Edit summary located under the main edit box and above the Publish changes button:
The edit summary field appears above the "Publish changes" button
It is good practice to fill in the edit summary field, or add to it in the case of section editing, as this helps others to understand the intention of your edit. Edit summaries are displayed in lists of changes (such as page histories and watchlists), and at the top of diff pages.
It is considered good practice to provide a summary for every edit, especially when reverting (undoing) the actions of other editors or deleting existing text; otherwise, people may question your motives for the edit. Accurate summaries help other contributors decide whether they want to review an edit, and to understand the change should they choose to review it.
Edits that do not have an edit summary are more likely to be reverted incorrectly, because it may not be obvious what the purpose of the edit was. Editors should not revert an otherwise good edit because of a missing or confusing edit summary; good editors may simply have forgotten, or a confusing edit summary may have been the result of an autofill mishap. (If the edit summary itself violates privacy or other policies, see the #Fixing section below.) However, realistically, when a major edit (e.g., addition or deletion of a substantial amount of article text, or a substantial rewrite) doesn't have an edit summary, some busy editors might not assume good faith and revert the change without evaluating it properly. Providing an edit summary helps prevent that kind of error.
Summaries are less important for minor changes (which means generally unchallengeable changes, such as spelling or grammar corrections), but a brief note like "fixed spelling" is helpful even then.
To avoid accidentally leaving edit summaries blank, you can select "Prompt me when entering a blank edit summary" on the Editing tab of your user preferences, if you have created an account.