Search Engine Optimization (SEO) is the process to top rank your website in search engine's. It's unpaid results—often referred to as natural / organic results. If you want your website to appear on the first page of a search engine when one searches for keywords of your interest then you need SEO.
People using search engines to find products, service or information they are interested in. If your product, services are not visible to them, then maybe you lose the customer. So it's important that your website appears as high up the search results as possible.