A JSON schema is a separate JSON file that describes every fields found in a LDtk JSON file (see official JSON Schema website).
That includes field types (string, integer, object, etc.), their requirement state (ie. required or optional), and descriptions. A schema is extremely useful to write importers and could also be used to validate a LDtk JSON file. Some tools, like QuickType, can even build a fully typed importer from just a Schema file.
The LDtk Json schema is also always up-to-date, as it’s generated right from LDtk source code.