Extra code on your page that tells Google exactly what type of content it is.
Schema markup (usually JSON-LD) tags your page so Google knows: this is an article, this is a product, this is a recipe, this is a review. Schema can unlock rich results: star ratings in search, recipe cards with cooking time, FAQ dropdowns right in the SERP. Doesn't directly boost rankings, but dramatically improves click-through when it earns a rich result.
Schema is underused. Most sites don't implement it and leave rich results on the table. For any content type that can earn a rich result (articles, products, recipes, events, FAQs), add the schema. The effort is small; the SERP real estate boost is real.