In more detail
Schema.org is a shared vocabulary created by the major search engines. Markup is usually added as JSON-LD, a block of code in the page that lists facts such as an organisation's name and logo, a product's price and availability, or an article's headline and dates. Google's introduction to structured data recommends JSON-LD.
Google supports a specific set of types for rich results, listed in its structured data search gallery, including Article, Breadcrumb, Event, Local business, Organization, Product, Recipe, Review snippet and Video. Google has reduced or retired some rich results over time, so check the current gallery rather than older advice.
The rules matter. Google's structured data policies require that markup describes content visible to users, is accurate and isn't misleading. Marking up fake reviews or hidden content can lead to a manual action.
Structured data doesn't guarantee rich results, and Google says it isn't a general ranking boost. It helps Google understand content and makes pages eligible for enhanced displays. Google also says no special schema is needed to appear in its AI features.
You can test markup with Google's Rich Results Test and the Schema Markup Validator.
Why it matters when hiring an agency
Structured data is often recommended in audits. Ask which types the agency would add, which rich results they make eligible for, and whether the data comes automatically from your CMS or is entered by hand. Hand-maintained markup tends to drift out of date.
Be cautious of agencies that promise rankings from schema alone, or suggest review markup for reviews you wrote yourself. Self-serving reviews of your own business aren't eligible for review snippets.