JSON is inherently schema-less. It’s not relational. It’s not the kind of structure that makes up the “S” in “SQL”. It’s more free-form. It’s nested.
Sometimes, however, there’s a need to constrain (or just describe) what shapes a particular use of JSON are allowed to take. JSON Schema is a IETF Standard & language (with specification) for doing just that.
JSON Schema is used in the OpenAPI standard, as well.