IR2101 requirement of Bootstrap Capacitor

Can somebody help me understand the requirement and importance of Bootstrap Capacitor in the IR2101 gate driver circuit? is there any relation to Under Voltage Lock out function with the Bootstrap Capacitor? Why is it considered to HIGH side??

I didn't have the chance to use a high/low MOSFET driver (as IR2101), so I know about the subject by analysing the circuit only.

When the high N-MOSFET is off, the pin Vs is likely close to ground (via the load) hence the Bootstrap Capacitor is charged (see pin Vb) at Vcc (actually at Vcc - Vdiode).
Then when it is on, the voltage of Vs will increase making the pin Vb also at a higher voltage than Vcc. This lets the voltage at pin HO for the gate be higher than Vcc too, and this ensures the turn ON of the N-MOSFET.
So at each transition of Vs from 0 to Vcc (ideally), the bootstrap capacitor is recharged to about (Vcc - Vdiode + Vcc).

Finding the optimimum value for this capacitor is likely related to Vcc and load for a few.
I never did its calculation (since I didn't built your circuit yet) but there are, for sure, some good refrences for it.