APIs are an interface that allows two disparate chunks of software, services, or platforms to transact with each other through a request-response message system. An API schema is like a database schema definition but for APIs, to make integration between platforms easier for developers. A schema is metadata- data about how data is structured. This is important information for a developer wanting to interact with an API and its endpoints.
API schemas provide machine-readable structured documentation that, together with regular documentation, make an API more discoverable for developers. Developers validate schemas to ensure they define what operations can be performed and they are within specification.
When a developer wanted to integrate a web API with another system or application, they typically had two choices. Either laboriously write the code by hand, or use an application integration platform, often referred to as an iPaaS. This included using a connector (again, written by hand) to wrap the API, or a software development kit, also called an SDK. Both options were time-consuming.
JSON schema are defined in the IETF JSON schema document. These are objects written in JSON that describe JSON schema. Validation checks ensure syntactic and semantic adherence to the JSON specification.
In the early days of REST APIs, there were competing specs for schema and documentation. Since Swagger was donated to the Open API Initiative, it has become the de facto standard of API schema. It is an extension of the JSON schema, and can describe an entire API. Standardization has brought increased tooling, with documentation, validation, and integration becoming a much more automated process.
API schema helps developers interact with an API, and the standardization of that schema has made the integration process much more automated.