What is the Atom "feed/entry/summary" element? How To Generate a "feed/entry/summary" element?

The <summary> element is not a required sub-element of the <entry> element. But it is strongly recommended that you provide a summary element for each of your feed entries. You should follow the same rules on Web page description meta elements to generate your Atom feed entry summary elements.

One strategy is to use the same value of the description meta element as the Web page that your Atom feed entry is based on. For example, if one your FAQ page is selected as a feed entry in Atom feed document, That Web page has the following description meta element:

 <meta name="description" content="A collection of
  16 FAQs on Atom feed file standard. Clear answers are 
  provided with tutorial samples on introduction to Atom 
  feed file standard; various ways to generate Atom feeds; 
  linking Atom feeds to Web pages."/> 

You should simply borrow value from the description meta element of that page to generate feed entry summary element:

 <summary>A collection of
  16 FAQs on Atom feed file standard. Clear answers are 
  provided with tutorial samples on introduction to Atom 
  feed file standard; various ways to generate Atom feeds; 
  linking Atom feeds to Web pages.</summary>

⇒Atom Specification - RFC 4287

⇒⇒RSS FAQ and Tutorials