When you're website comes up in search results the title of your page is followed by a brief description, as seen below:
You want the description of this page to provide people with a message that will cause them to visit your website. You can specify what this message says and include marketing-oriented language that gives a good indication of what they'll find on that page.
Navigate to the page you want to add a description to.
Select edit, then metadata.
In the description field of the metadata section, type your description for the page. Try to use relevant keywords that people might search for and 150 characters or less. Be sure to use a different description for every page on your site.