The Linux kernel simplified
What is a kernel in “Jip & Janneke taal (Dutch saying, simplified). A kernel basically makes sure all hardware and user programs/applications can work together.
A kernel points memory and processing time to your programs/applications. A kernel makes sure your keyboard shows the input in your Gedit or Vi programs. (both text editors).
A kernel also makes sure, your mouse pointer moves on your screen when you use your mouse or when you tap on your mobile’s device screen, the kernel will handle your actions 
Android uses the Linux kernel as well 
Android uses Linux
Simple picture of the kernels work:
With Linux, you can even compile your own kernel.
Maybe you only need certain drivers or you want to run Linux on small device like Rasp Berry Pi’s, and you don’t need a full running desktop environment, just a running kernel with a minimalistic shell.
Compiling your own kernel can look like a big step, but when you’ve done it a few times, it’s not that scary anymore.
More information about kernel compilation: Right here