IRS2110 IC & high side floating supply voltage - H bridge
The root of the problem to be solved by this circuit is that the Drain of the upper FET is not at a fixed voltage level (goes up &amp; down), and you have to supply about 10V relative to its Drain at the Gate to open it.


  What exactly the bootstrap capacitor is doing over here ?


It creates a voltage that is about (almost) VCC + VS. Since VS is sometimes almost equal to the highest power supply line within the circuit (up to 500V or 600V, according to the drawing in the data sheet), there would be no way to get a voltage ~10V higher than that without some kind of "magic". The magic works by charging a capacitor through a diode to about Vcc when Vs is at GND (lower FET is conducting), and then this capacitor can provide the neccessary voltage level needed to open the Gate of the upper FET, and keep the Gate at a higher voltage even when the upper FET opens and VS goes up.