Many new users leave this line blank. That is a serious mistake. The page history, recent changes and related changes all report this line and filling it in can save a lot of extremely annoying clicking and peering to see what changed. An accurate one-line edit summary is actually more than enough to report most small changes verbatim, and some larger changes as well.
 edit summary tag
Mediawiki supports links in the summary line to make it very easy to go to the relevant pages to look up the motive for the change, verify facts, and etc. An edit summary tag is a cite link used in an edit summary to say "here's why I'm changing this". As with any edit summary such a tag helps other users look up exactly why a change was made, which reduces editorial disputes. For instance, if you have removed unnecessary capital letters or anchor text, adding [[conserve capitals]] or [[avoid anchor text]] lets others look that policy up quickly.
Common tags include rm spam, tyop and tangop. These speak for themselves, admittedly in trollish. It's more common to write in English and use phrases like "add category..." or "...for now" within the sentence.