Ahhhh yes meta tags. I remember you so well. Just a few years ago (ok many years ago) all you had to do was stuff meta tags with keywords and you would rank #1. But no longer are they that powerful. Still some meta tags are good and others are just worthless. Here is a rundown of metatags.
Newbie Note - In case you don't know what a meta tag is, they are part of the code of a webpage. You enter then at the top of your code in the header. They are mostly never visible to users. To learn more basic info: http://en.wikipedia.org/wiki/Meta_element
Title Tag - Technically not a meta tag but it is important. The title tag is a valuable factor that search engines use when ranking a site. Make sure your keyword is in there. Do not keyword stuff. The title tag is commonly used as to represent your site in the search results. Make your title something that will attract people to click on it (just don't forget the keyword).
Meta Description - Search engines still look at this but only place low to medium value on it Ideally the meta description should be unique and relevant for each page containing 1-3 keywords in it. It should be about 1-2 lines ago. Search engines sometimes use your meta description in the search results so remember to make it attractive to the user so you will get more click throughs. Search engines do not always use the meta description as the snippet in the search results sometimes they replace with a more relevant snippet of text from your page. It is not necessary but does provide some ranking help.
Meta Keywords - Some search engines have publicly stated they ignore the meta keyword tag. There is no to minimal benefit with this tag. This is because too many people have abused it and it has become useless to search engines. I sometimes use meta keywords because in isolated cases it can help with duplicate content issues. In general this is a dead tag and should probably be skipped. If you still have meta keyword tags you should review them. Some search engines penalize you for keyword stuffing that might have occurred by the previous webmaster many years ago (when it used to be a good idea).
Meta Language - This was a cool tag. It was supposed to identify what language the content was written in (English, Spanish, German etc). Unfortunately like the meta keyword tag it was not properly used by webmasters. Webmasters started exchanging free templates and forgot to change the meta language tag or forgot to even add it. That has led to a large amount of websites with inaccurate or missing meta language tags. That is why most search engines do not look for this tag.
Meta Robots - This tag is intended to help control the automated bots (like search engine crawlers) on your website. This tag is still observed by all major search engines. Even though this tag does work and can be helpful I would suggest you use a robots.txt file instead whenever possible.
Metas Not Mentioned Here - I tried to mention the most common ones but did not have time to go through every meta tag. If you have come across a meta tag that seems weird or too good to be true it probably is. Since search engines mostly ignore meta tags, some webmasters have used it to play jokes. For example many years ago some people started using "meta googlerank=#1" as a joke. Then some ignorant webmasters blindly followed and copied these fake meta tags onto their site and thus the myth began that if you just add a special meta tag you will rank higher. Unfortunately there is no magical meta tag.
Meta tags are part of the complicated ranking formula but a very small part. Visible content and good links are where I would suggest you focus your efforts on. They have much greater SEO rewards.