Google: Web Page Description
A page's description meta tag gives Google and other search engines a summary of what the page is about. Whereas a page's title may be a few words or a phrase, a page's description meta tag might be a sentence or two or a short paragraph.
— Google: Search Engine Optimization Guide
The description is in the markup: <meta name="description" content=".......">
- The description is very important, because it's what Google will display about your webpage, and people will decide to click on your link based on that description.
- You don't have to provide a description, but... Google will generate one for you and you will also be penalized in the page ranking.
- Words in the description are bolded when they appear in the user's query.
- Use unique descriptions for each page.
- Do not use generic descriptions like "This is a webpage" or "This page is about ..."
- Do not fill the description with only keywords.
- Do not copy and paste the whole webpage in the description meta tag.