SEO (Search Engine Optimization) is the practice of improving your website so it ranks higher in search engines like Google. Higher rankings bring more organic (unpaid) traffic to your site.
SEO means optimizing your content, technical setup and authority so search engines understand and rank your pages. The goal is to appear for the searches your audience makes.
SEO itself is free, but it takes time and effort. Paid ads are separate from organic SEO.
Typically 3-6 months to see meaningful results, depending on competition.