The practice of ranking higher in Google's organic search results.
SEO is making your website show up when people search. It's a mix of technical (Google can crawl and understand your site), content ('the right' content to rank for your target keywords), and authority (other sites link to you, which Google reads as credibility). Organic traffic from SEO compounds, which makes it one of the most valuable long-term channels.
SEO is the cheapest customer acquisition channel once it's working, but it takes 6-18 months to build. Most local businesses and content sites should take SEO seriously as a long-term play. Ignoring it for short-term channels is a common regret.