SEO combines three areas: technical SEO (making sure search engines can crawl and index the site), on-page SEO (content, titles and structure that match what people search for) and off-page SEO (links and mentions that build trust).
Unlike ads, SEO traffic does not stop when a budget runs out, but results build over months rather than days. Progress is measured with impressions, clicks, rankings and — most importantly — leads and sales from organic search.