Search Engines rank pages for a given search term using two things... the relevancy of the page to the search term and the importance of the page overall.
- Relevancy is determined using hundreds of different factors such as page contents, title tags, headings, alt tags etc.
- Importance is interpreted as popularity by the search engines and again is determined using hundreds of different factors, the main one being incoming links which are seen as votes for the website.
Even if your pages are highly relevant to certain terms you may not be realising your full potential because you are targeting the wrong terms. You want to target terms that your potential customers use to search for your product or service but you also want to target terms that don't have strong competition.
Improving the importance or popularity of your pages and website could be more difficult and will require good quality content and search engine optimisation.
source: SEOmoz