Meta tags are the hidden codes on your website that help the search engines correctly identify the content on the pages so that they can deliver the most relevant results when someone searches.
There are many meta tags (almost 30), but the three most important are the title tags, the keywords and the description.
The title tags are what describe each page like a headline in a newspaper. These tags will be seen at the top of the web browser, and are the first line in the results of a search using a search engine like Google, Bing or Yahoo.
They keywords are a list of words or phrases, separated by a comma that describe the topics your website covers. The more descriptive the keywords, the more likely someone looking for your business will find your website. One additional element to keywords is to try to narrow the scope of your keywords to keep the number of competitors down. It won't do you any good to have a keyword so broad that there are billions of competing web pages with the same keyword. Consider cars, for example. That's a very broad keyword. Perhaps a better keyword phrase would be "race cars." Or even better, "hand-built, custom-spec race cars." The narrower the search term, the more likely your web page will be found.
The description meta tag is a short description (under 160 characters) that displays right under the title tag (headline) in the search results. This isn't the place to sell your product or service. Think of this as your chance to entice people to click through to your website.
One additional tag that merits a mention is the Alt tag. Alt tags are attached to images. Search engines can't see the images on your web pages, but they can read the alt tags. Anything that helps to make a search more relevant will help the search engine ranking for your page.