A meta description is a short snippet of text that appears in the search results under the page title and url. The meta description describes the content of the page and should encourage the user to click by assuring them that the page contains the information that they are looking for.
The above is a search results snippet for the SEOniverse homepage. The purple text is the page title or meta title (the user in this case has clicked on the link before, otherwise it would be blue) the green text is the url and the dark grey text underneath is the meta description.
In the <head> section of your HTML webpage tag the meta description tag should go directly under the <title>. It should looks like this:
<meta name="description" content="The text in here is the meta description">
Be aware that there is a limit on the length of your meta description. This is best understood as a character limit but is more accurately based on the number of pixels in your description. If you exceed the maximum number of pixels the search engine will cut off your meta description early with an ellipsis, as in the example below:
Capital letters have more pixels than lower-case letters so a meta description written in capital letters will be permitted less characters. However, as a rough guide try to a maximum of 160 characters - anywhere between 140 and 160 characters is usually the optimal length for a meta description.
Some sites have hundreds of thousands of pages and crafting unique copy for every page would be an ultimate waste of time. It's a good idea to spend time perfecting the copy on your main pages but further down the hierarchy it's perfectly fine to rely on Google creating a meta description based on your on page copy.
Yes! Even though the meta description isn't taken into account by Google's algorithm, the keywords in the description that match the user's query are bolded. Take what you can when you're trying to get your description to stand out from the competition.