Meta tags are very important to your website because the search engines display these informations during a search. The main ones are the title, the description, and the keywords.
The first two doesn't require explanation, so let's talk about keywords a bit. When you are searching you are trying to think about small pharses or terms, such as "McDonalds New York". You aren't searching for "I'd like to eat in a McDonalds in New York". So you are using keywords. Each of them has two main attributes: how many people are seeking for them, and how big is the competence on it. You can check the ones you need at
Google Keyword Tool.
How to choose keywords?
Well, don't target the most searched ones first. The huge numbers are temptating, but there are millions of sites out there. Try to be more specific with them. If you are running a printing company, add your location to your keywords too like "printing services New York". Also, if you are running a gaming forum about World of Tanks, don't target the keyword computer gaming, instead, choose "World of Tanks Forum".
Here you can generate meta tags for your site. Just follow the instructions.
Click!
When finished, copy and paste the generated code to your html files, between the < head > and < / head > tags. (without spaces)