Structured data has an explicit data model
Structured data is data organized according to some particular data model, which explicitly defines the structure of the data. This is usually a database or strict spreadsheet format.
Structured data makes it possible for automated processing to make assumptions about what the bits of data mean and do processing on them, for example calculating date ranges. Computational processing of data in these formats is much easier than on unstructured data, even if all the same information was present in both formats.