In plain language SEO or Search Engine Optimisation is making sure that you get the key elements of your web pages right for the way different search engines look at your content. Google, Bing and Yahoo (to only name 3 popular search engines) will look at your website and evaluate the quality of its pages and this will affect your position in the natural search pages.
The key to using the search engines effectively is to develop an understanding about the type of keywords and phrases used by your customers.
Using the Drupal Content Management System you can be sure that your site is build in a completely accessible way,a and that you can spend time and energy on getting the content and keywords just right.
For more information of Search engine optimisation watch http://www.youtube.com/watch?v=hF515-0Tduk.