SEO is an ever-changing algorithm Google morphs every so often to make you aware of the components necessary to get your website ranked in a top position within the organic (free) listings.
Google is, unfortunately, fairly secret about this algorithm but word spreads rather quickly concerning the changes, meaning you need to stay on top of what's happening in and around the SEO watering hole.
There exists NO simple answer to SEO although I can tell you what it means and I can explain the results of a successful SEO campaign.
1. Find your most appropriate keywords or keyphrases through research done in Google or a pay for info site.
2. Incorporate those keywords in each page of your website in your page copy.
3. Add a meta tag description using your newly-discovered keywords or phrases in your code and position it between the 'head' and 'body' sections.
4. Title each page with different phrases.
5. Change your website twice a month - preferably by growing the site; always adding new and informational copy.
6. Flash animation and images DON'T COUNT! The Google spiders can't and don't read flash animation and images. By all means insert them for looks, but forget about them making a difference in your score... they don't!
7. Finally, make your website useful and well thought out without overusing your keywords and keyphrases. 3 times per page is enough.
As you can see, there exists NO simple SEO explanation. In fact, I made my answer as simple as I could, making sure I included the main areas.
All that to say, unless you are willing to spend many hours studying SEO and staying current with the changes, it's best to hire a pro to handle this for you. "Pay me now or pay me later," is an old Pennzoil slogan, but it fits this constantly changing scenario of SEO.