We're rolling it out slowly. When you open a document, you can view a summary—which you can scan to see what topics are in the document. If you're the author of a long document, you can use the summary to quickly see if the content is organized the way you want. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.