Alongside of the correct use of your meta tags your site’s content is the most important aspect of your website’s search engine optimization.
Your content is what the search engines crawl through and determine what your site is about. So, for example, with 20 well written articles containing your chosen keywords on the topic of car stereos the search engines know instantly the area your your ‘expertise’.
Also, it is your potential customers or site visitors that are interested in your content. They will want to read informative, up-to-date and unique articles.
It is now, after mentioning the above two scenarios, that they start to merge together.
Due to the fact that your website has a constant flow of information being added to it – articles, videos, photo galleries, Twitter and Facebook updates, etc – the search engines say “Hold on, this is no spamming website. These guys are putting in some effort, they must be for real”. Then your content starts drawing in more visitors (both new and regular readers) and your site becomes popular which feeds the search engine ‘fire’ some more “Wow, these guys are getting lots of hits, they must be good, let’s put them higher in the results”. And because you are higher in the results more people visit …..and so on and so forth.

An important point to note.
Above we mentioned these following terms to describe your content and we will now look at them in depth -
Well written – the search engines don’t like spelling and grammar mistakes, neither do your visitors (I guess this site is a failure then, ooops!).
Informative – if you are going to write about a new car stereo then actually write about it, not just say “It has been released!”.
Up-to-date – you will not attract visitors by writing about old news. Check out these alerts and keep current.
Unique – search engines do not give you credit for ‘duplicate content’, so don’t think you can create a huge awesome website by copying and pasting. Quoting other peoples work is fine but there is a line which you shouldn’t cross.
Chosen keywords – a general rule is that your chosen keyword should be mentioned three times within the article and the article be at least 300 words long (this is called keyword density). This is a huge generalization but it is a good place to start until you get the hang of it.