What is a kernel?

Most of you guys already have heard 'KERNEL' many times but do you know what it is? 
If not then this post is for you

All Operating Systems like Android ,Windows,iOS etc have kernel .But we guys are interested in Android devices which basically uses Linux kernel

Kernel is basically a link between Software and Hardware. The kernel decides when things happen.
 e.g. 1.  the LED indicator gets lit. An application sends a request to the operating system to blink the LED. The operating system then sends the request to the kernel, which makes the light flash for the amount of time requested by the OS. 
2.Controls the brightness of the screen &  volume level etc etc etc i.e. Everything. 
Therefore Application sends information to kernel which then passes to hardware.
It has to be stable, Friendly but it is  incredibly difficult for the layman to modify.Its complicated but its  standard computer logic. Kernel is loaded everytime device boots.
So we use Custom Kernel to change the clock speeds to make the processor go faster or slower.