CMake defines the location, arguments, and properties of build tools using toolchain files. In short, these files specify what configuration of tools to use to build the CMake project. Since CMake toolchain files can be shared between projects, F´ mostly uses them as-is. To set platform specific fprime settings a parallel Platform file may be created in order to keep those settings out of this more generic file.
See: fprime Platform Files See: https://cmake.org/cmake/help/v3.12/manual/cmake-toolchains.7.html
To create a new toolchain, copy the toolchain.template.cmake file and fill it out. Then create a platform file in the platform directory. Other CMake toolchains (from CMake, other CMake based projects, etc) can be used. The appropriate platform file must be created, if it does not exist.
To understand the toolchain template: Toolchain Template File To use the template: fprime Toolchain Template