You'll see a summary available when you open a document and you can scan it to see what topics to expect. 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. The summary will be in a collapsed or partially open section at the top of the page. 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.