When you get online and search for a topic you use search engines to facilitate your research for information, right?
Now how do these search engines (out of over 20 billion pages on the web) decide which pages would answer your query? They pick out the pages with the key phrases or terms you've entered in the search engine. Join Matt Cutts (click on the video to the right) as he explains in detail how Google Search works. Now, when you get the list of your search results adding up to say, 930,000 pages, you are only going to visit the websites at the top of the list and if you're very persistent or desperate for information, maybe look over the first three pages of the results, ignoring the last 929,997 pages.
Search Engine Optimisation is the process of getting search engines to find and rank sites high on the search results in response to a query. Search Engine Optimisation directs more traffic to your site, more visitors, more potential clients which ultimately means better, bigger, improved business for you.