Next to the field name is a link that reads Edit Summary. When creating a content type, the creator decides whether the body text field should accommodate having a separate summary, and if so, that link is shown. The summary is used, for example, when providing a list of content for the site visitor, where a short description of the content—a teaser—is provided.

There are two ways to provide the teaser text: via a separate summary or by trimming, that is, excerpting text from the start of the content. I prefer to use the summary because trimmings are based on a set number of characters that is the same for all of the same type of content, such as the first 40 characters, so I have no control over where the excerpt will end... maybe it will end in the middle of a word. With a summary, I can precisely control what will be shown. Even if it is identical to the start of my body text, I am able to ensure that it ends in an appropriate place.

Click the Edit Summary link. A Summary text box will appear above that for the body. It contains a link to hide the field again. This toggle of Edit summary/Hide summary allows you to remove its distraction until you need to use it. You will notice that the bottom right corner of the text box has a widget that's meant to resemble page corners that can be grabbed with the mouse and dragged to enlarge or reduce the size of the text box. It also has a note below it letting you know that if you do not enter summary text into the field, Drupal will use a trimmed version of the body text as a summary. Enter some summary text into the field. Mine will read: Sasha makes an exciting discovery.