Structured Data (also called relational data) is data that lives best in a structured format, i.e., the kind of data you would enter into a table in a spreadsheet. The best way to organize it is in columns and rows.
Two examples of structured data are an inventory of products for an eCommerce site, or a list of customers and their information. CockroachDB was designed primarily to support this kind of data (although it does also include support for unstructured data).