Schema Markup is structured data vocabulary, often from Schema.org, used to describe content in a standardized way.
It matters because technical issues can prevent great content from being discovered, rendered, indexed, or trusted. Strong technical foundations protect rankings during redesigns, migrations, content expansion, and platform changes.
Example
An event page can describe the actual event date and location using relevant Schema.org properties. The structured data should match the visible page, and a valid property value does not itself guarantee a rich search result.