Summary: This should be a brief, concise description (summary) of the content on each page. This is the summary that will be displayed in search results on Google for example. (those 1 or 2 sentences under the title of the search result).
Descriptions should be accurate, concise and should contain words that both reflect the content n the page as well as the terms people use to search for and find you pages. It can be helpful to carefully craft in your keywords as well.