SEO stands for Search Engine Optimization. It is extremely important because this is how you generate organic traffic to your site. Organic traffic is when your page appears at the top of search results. You can tell how well your page has been optimized by seeing how it ranks in a search result.
Google (or any search engine) has a crawler that goes out and gathers information about all the content they can find on the Internet. Those crawlers compile what they see in your website and try to match it with customers. Google is in the business of getting the right information to the right user. SEO is all about making sure that those crawlers see what you want your customers to see. We want to make sure that Google or other search engines show your site as the number one authority on that topic or service.
What makes SEO important?
If your site is not at the top of the first page your page has not been optimized for that specific search query. Unless you are paying for ads that match that query you could be missing out on a lot of customers who want your services. Remember, when was the last time you went past the fourth or fifth result when you were searching for something?