SEO comes from Search Engine Optimization: optimization for search engines. In simple words, it is the set of tasks that make your site appear when someone searches for what you sell, without having to pay for an ad.
The difference with advertising is simple: the ad stops bringing you people the day you stop paying for it. Organic positioning keeps working months later. It is slower to achieve and much cheaper to maintain.
And there is something new that almost no one is addressing yet: more and more people are not searching, they ask . To ChatGPT, Gemini, Perplexity, or the AI summary that Google puts at the top of everything.