If the capacitor were left out, you see the classic gain-of-one, non-inverting amplifier. The input impedance of the circuit would be the parallel values of the bias resistor to ground (in this case 200K) and the very high input impedance of the amplifier. When the capacitor is added, the AC input impedance of the circuit rises to a very high value because the bottom end of the upper bias resistor now "wiggles" in synchronization with the output of the amplifier. Hence, no signal current is impressed across the 100K resistor while the pair of resistors maintain a DC reference for the amplifier's input pin. The bootstrap makes the DC bias network 'disappear' with respect to applied signal.
