Hi,
If you are aware of the Analogue and Digital electronics then it will be very easy to understand, anyways:
In analogue circuits take an example of an Audio amplifier, when you are listening music and in between get a noise ripple it will simply disturb the music quality for a short time and will continue, simple.
But in Digital circuits if a noise is superimposed on DC supply and it enters the Microcontroller (which is purely working on the Lows and Highs) may not ignore this noise and will change some of its function as output which is not desired at all. Just to eliminate this Noise on the DC supply a Decoupling Capacitor is used, as the AC ( a noise in this case) can pass through the capacitor and DC remains blocked. Here with the Decoupling capacitor (some times called Bypass Capacitor) the noise passed to the return path (Ground) without disturbing the working of the Microcontroller.
This is in short why the decoupling capacitors are used.
Hope it explains a bit.