Legion

In short: The center of your system. What talks to the hardware. What makes a user program not have to worry about details. What makes the rules. The boss. The kernel.

Like I did, you may at this point wonder what the job of the kernel actually is.
For a beginner it may be quite a gray area what goes where and why. But the kernel makes sure the computer works. It makes sure characters appear on the screen. It makes sure that the file “hubba.file” is found (whatever that means in your system) and read correctly when requested by a program.
It's the extreme basics - and you will have to write it.