In the most specific understanding, Search Engine Optimization (shortened to SEO) has to do with improving how well your website is displayed in search engine rankings. As a business owner, you are not being asked to become SEOs, which is a field that requires broad knowledge. After all, your website developer will already have done SEO by setting up the website right and doing the basic listing with Google and Bing.
What you as a business person need to know is there are things that you can do to improve search engine rankings without spending any more money:
Once your business displays well on search engine results pages, the number of people visiting your site will improve. And the more people who see your site, the more will walk through the door and become customers.
If you are looking for information about growing your business with advertising expenditures, then see Internet Marketing.