Selected answers from the Dumb SEO Questions Facebook & G+ community.
Christine Hansen: A Meta description is a page description, where you tell your visitors what the page is about in 155 characters, which is shown in the Search Engine Result Page (SERP).
Write ad-like useful Meta descriptions with a focus on benefits and values and use one or two keywords. Your goal is to make your text clear enough to receive a better Click Through Rate (CTR) - You want to give readers a reason to click.
If you leave it blank, you leave it up to Google to insert something, that does not always look nice or makes sense. Text often gets cut off missing important details.
Every Meta description has to be unique - do not use duplicate descriptions.
Stephen Kaufman: Meta desc should be your sales pitch on why your link should be clicked on. Also google highlights searched keywords in the meta desc, so yes its good for them to be there.
Daniel Dutton: Meta descriptions have never really had much of an impact on rankings but could be used to entice people to click to your website because it was shown under the title in the serps. These days, google puts whatever excerpt it wants under the title so a lot of the time the meta descriptions is moot. I haven`t bothered writing them for years.
Casey White: They don’t affect ranking but keywords matching Title Link (Tag) often bold. Still good to include, but write them more like a pitch.
Trenton Erker: Britney Muller showed at Moz Con one year that descriptions do impact snippets.
However, nothing shows that descriptions help rankings and Google says it out right that descriptions don’t help rankings.
The reason to include a keyword in the description is to appear relevant to the searcher’s query.
Also, words in the description that match the query are emboldened meaning they stand out.
Stacey Colthup: Thank you everyone, this all makes perfect sense!
Muhammad Aizaz Shahid: its does not affect ranking but its makes a impact on the users that they should click on the blog post on not use unique description or make the user curious
I was just told having your key words in your meta descriptions won’t affect your ranking so don’t bother putting them in… Is that right? 
