SEO stands for Search Engine Optimization, which is the practice of increasing the quantity & quality of people coming to your website through organic (“unpaid”) search engine results.
Quality traffic is really important because you can attract all the visitors in the world, but if they’re coming to your site because Google tells them you’re a resource for Apple computers when really you’re a farmer selling apples, that is not quality traffic. Instead, you want to attract visitors who are genuinely interested in the products that you offer.