[alok9shm]

If you knew what is a microcontroller, you should not have asked this question. So, you should know what a microcontroller is. You google that out for the exact definition.
I'll give you the rough info.

Its an embedded system in itself, i mean to say, a processor, a memory, etc all in a single module. For example the microcontroller chip in tv remotes, toys, digital appliances, etc.
Now take the example of a tv remote control. If you press 1, the remote sends a specific signal to the tv, pressing mute, will send a signal to the tv that mutes the volume, other buttons do specific jobs. Now, this thing needs to be programmed in the memory or ROM of microcontroller present in the remote control to make it work the way it does. And this programming is what you call Microcontroller programming. Hope you got the idea.
Roughly, microcontrollers are a small size computers (you can call it a computer, isn't it?) that does only a specific task. Like, in the case of Remote controls, it sends out signals corresponding to the pressed buttons. That all it does. So, its a kind of small size, limited/specific function, computer, that takes input, processes it, and gives the output. It contains a ROM, where we need to feed the instruction regarding how this device will work. This set of instruction is a program code [in C, PASCAL, BASIC, or Assembly etc]. Mostly it remains unchanged in a device. Writing these programs into the microcontroller ROM is Microcontroller programming.

And why is it necessary? Do I need to tell that too? A device will not work if it doesn't have a program that tells it how to work. To make an electronic device work, a program is required, and thus comes the need of microcontroller programming.