The Cayenne (REST) API allows developers to create custom web and mobile applications that are driven by Cayenne without the limitations/design choices of our first party web dashboard/mobile apps. By using our API in your code you could do things like interact with sensors/actuators connected to Cayenne and access our scheduling, triggers, and alerting system from your own app.
We have a developer working on a sample custom app that we'll have in place at that link soon, so you can get a better feel for what the code for such an app would look like, and we have API documentation here.
If you have some specific question about building your app with the Cayenne API, please feel free to create a thread about it on this forum or post to our public Slack channel for advice and help. We'd love to have your feedback and hear more about what you're hoping to build with it!