A meta description is the black text displayed underneath a link in search results:
This text is entirely optional, in the sense that if no text is entered search engines will generate them. However, it is unlikely that automatically-generated text will be as attractive to searchers as one written specifically to entice visitors. Take a look at two examples that rank in the UK for the keyword "shopping":
Words used in descriptions do not count towards rankings, but have a significant impact on clickthrough rate. For this reason, an ideal description will contain a call to action and explain benefits to encourage more people to click on our listing, as opposed to others on the same page of search results.
While you do not necessarily need to include keywords in the description, they are two reasons why you should:
Because the goal of a description is to increase clickthrough rate, it is pointless to create one if it is unlikely to achieve that objective. Simply having a meta description is not an advantage in and of itself.
As with a title, descriptions are not displayed when a user lands on the site, but instead are only seen on search result pages. Thus, they should also make sense out of context. We recommend writing as if a third party was describing the page, as this results in a higher clickthrough rate.
Descriptions also have a very strict limit on the amount of text you can enter. Users will never see text that is 'cut off' in a description, and so keeping things short is vital. As a rule of thumb, we recommend a description of between 20 and 25 words.
Include a Meta Description of approximately 20-25 words that encourages users to click by using a call to action.