When editing a page, a small "Summary" field under the main edit-box looks like this: Image:Edit Summary-2.png It is good practice to fill in the Edit Summary field, or add to it in the case of section editing, as it helps everyone to understand what is changed, such as when perusing the history of the page. It's a good idea to set your user preferences (under Edit) to "Prompt me when entering a blank edit summary".