From the Kepler Overview page in our developer docs
Development Platform
Kepler is a new SDK from Amazon that you can use to create apps for Fire TV. The Kepler SDK provides the tools needed to create, build, test, package, and publish your apps.
Kepler Codelab
If you are new to React Native development, you can use the Kepler Video Player codelab to introduce you to key concepts and learn Kepler fundamentals. This codelab walks you through the basics of building Kepler apps with React Native.
Simulated testing
The Kepler SDK includes the Kepler simulator, which is a virtual development platform you can use to test your apps. The Kepler Simulator allows you to see how your video apps will look on a Fire TV.
Head over to the Getting started category to quickly get up and running.