The Linux Kernel
Many refer Linux as an Operating System. Linux is not an Operating System. GNU/Linux is.
Linux refers to the Kernel of an Operating System. Much like the brains that control and interact with various hardware. The kernel systematically work in the background ensuring that there are no hardware conflicts, resources are effectively allocated and removed, and ensure that the hardware is able to interact with the software. There are more intricate features that the kernel do which I will not go into detail.