Search Engine Optimization is possibly the hardest beast to "tame" Google's algotithms are changing every minute, and what works today, may not work tomorrow.
If you have a product or service that you would like to sell online, or even attract local customers, then you should have some form of SEO implemented. If search engines cant find you, and be able to tell what it is that you do or sell, then your missing out on opportunities to gain traffic and sales.
Key Words / Key Phrases
Can the Search engines find you when you search keywords / phrases?
Is your Sitemap up to data and organized so Search engines can read it?
Have Ample Traffic to your site? Search engines know who/how many people visit your site.. the more the merrier.
How many other sites / pages link to your sites? Popularity plays a huge role in ranking results in Search Engines