The summary field is displayed as the shortened (teaser form) of your article. This will show up, for example, on a listing page of articles. It should capture the essence of your article and ideally be two to three sentences.
In elementum velit et nulla iaculis, vitae tempus nibh sodales. Mauris et ultrices nunc. Aenean eget convallis odio. Proin justo mi, pharetra id dapibus quis, luctus ut magna.