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.
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Maecenas tempus, nibh non finibus sollicitudin, ligula magna tempus nibh, ut maximus libero lorem ut sem nibh non finibus.
Vestibulum iaculis consectetur convallis donec sit amet congue massa. Quisque nec eros eu dolor posuere fermentum et vel nulla. Donec mollis leo sit amet egestas vulputate.