XNU is the computer operating system kernel developed and used by Apple in macOS. It’s been released as free and open-source software as part of the Darwin operating system. XNU is also used as the kernel for the iOS, tvOS, and watchOS operating systems.
In computing, the 'kernel' is the central component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources (the communication between hardware and software components).