The kernel is the engine of your GNU/Linux system. When switching on your box, the kernel is the first part of your operating system that gets loaded. It detects and initializes all the components of your computer's hardware, then makes these components available as a tree of files to the software, and turns a single CPU into a multi-tasking machine capable of running scores of programs seemingly at the same time. The kernel headers define the interface to the services that the kernel provides. The headers in your system's include directory should always be the ones against which Glibc was compiled and should therefore not be replaced when upgrading the kernel.