What is the use of decoupling capacitor?
Quite simply put, it's a little reservoir of energy for a chip or other device to dip into when it needs to get a little extra power.

In Electronics nothing happens instantly.  When a chip needs more power it has to get it from the power source.  The power source then has to respond to that by providing more energy, and that then has to make its way down the wires or traces to where it needs to be.

By placing a small capacitor near where the power is consumed the capacitor can respond to that call for more power much faster than the power supply.