An edit summary allows you to give a brief description of the changes you have made to the page. They should be used for every edit, except when creating a new entry, as they are exceedingly useful. (Do not use them when creating a new page: the software will fill in a useful edit summary automatically.) They are limited by the software to 255 characters.
Edit summaries help 
- Those who are looking at a page's to work out how the page has changed over time.
- People who have the page Watchlisted to see whether they are interested in your change.
- Anyone watching recent changes to get an impression of what sort of edits are going on.
A good edit summary 
- Contains a brief description of all the changes that were made.
- May include a brief reason for making the edit.
- Does not include excessive justification for the edit.
Abbreviations include 
- ISO 639-1 or ISO 639-2 language codes.
- The name of any templates used.
- any abbreviation listed at Wiktionary:Glossary or Appendix:Glossary