What is on-page SEO and what should I optimize?

On-page SEO involves optimizing elements on your pages that search engines and users see. Key elements include title tags (unique, descriptive, include keywords), meta descriptions (compelling, include keywords, under 160 characters), headings (proper hierarchy, include keywords naturally), URL structure (short, descriptive, include keywords), image alt text (descriptive, include keywords when relevant), internal linking (link to related content), and content quality (comprehensive, answers user queries). Optimize for users first, search engines second. Example: Optimizing title tags and meta descriptions increased click-through rates from search results by 15%, even without ranking changes.