Web APIs provide a way to expose Appian data and services to outside systems. Each Web API is an association between a URL/HTTP method combination and an expression. When a client makes an HTTP request to the given URL, the associated expression is executed and the result is returned to the client. This means that any data or service that is available inside an expression can be exposed to an external system via a Web API.
Some things you might choose to expose with a Web API include:
To learn about Web APIs, see Web APIs
To learn more about designing Web APIs, see Designing Web APIs