A meta description is a short description of a website page. Think of it as a mini-summary. You won't find this summary when you are actually viewing the pages, it lives in the HTML behind the scenes. It will tell a search engine bot/spider what each page is all about.
Meta descriptions are small, yet important piece any good search engine optimization strategy.
Now let me tell you this, but don't get too hung up on it...
A Meta Description will not help you rank better in search engines.
That's right. You can stuff as many keywords in there as you want, it's not going to improve your rankings.
When it comes down to it, your rank in a search engine isn't what is important. The traffic you get is what's important. Not only traffic, but traffic of people that are finding what they are looking for. Meta descriptions help with just that.
Your meta descriptions will pull into the search results just below the title. A good meta description can significantly improve click-through-rate (CTR). CTR is the rate your website result gets clicked in comparison to how frequently your website is shown as a result. The higher your CTR, the more website visits you'll get and hopefully the less your competition will get!
It's important to have a unique meta description so you have some control over what a search engine shows each time your website is shown in search results. If you don't have them a search engine will attempt to pull a portion of the page into the result and it won't always make sense or be easy to read. If your competitor's is, they'll likely get the click instead of you. It's almost like writing an ad for every page of your website; creating the message, capturing the attention of a searcher, and urging them to take an action.
There you have it. Meta Descriptions won't move you up in search results but they can make all the difference in improving the traffic to your website!