KDE Frameworks is a framework of libraries built on the top of Qt. The libraries are divided in 3 Tiers that define the chain of dependencies.
You don't need to use all the libraries: they are divided in a way that allow you to choose what you really need. For instance if you need to localize your application, use the Ki18n library, if you need an archive tool, use KArchive.
You can find the API documentation of the Frameworks on: apidox
If you want to get the most out of the Kirigami Components framework for mobile and convergent user interfaces, we suggest getting familiar with the Kirigami Human Interface Guidelines before using them.