Definition
Schema.org is the public vocabulary used to mark up entities (people, products, articles, organizations) so search engines and AI answer engines can understand them. Founded jointly by Google, Microsoft, Yahoo, and Yandex in 2011, it's the closest thing to a universal API the web has for content semantics.
Mark up your pricing page with Product + Offer types so engines understand 'Pro Plan = $19/month' as a structured fact, not just text.
How to optimize
Use the @type that best matches each page's purpose. Validate against the official schema.org vocabulary. Test rendering in Google's Rich Results Test.