Also known as Devkit, an SDK is software tools and programs used by developers to create applications. Think of it like any other kit, where it includes everything from documentation to instructions on how to build a particular thing.
An SDK has processes, code samples, and a set of other tools to help developers build applications on a specific platform. So, for example, if you’d like to create an iOs app, you’ll need an iOs SDK. The same goes for other platforms, for instance, Android SDK.