All op amps have a small bias current flowing into or out of their input terminals, for some op amps this is a very small current indeed. To enable the op amp to perform correctly, the circuitry connected to the op amp's input terminals must provide a path for these input bias currents to flow to ground, otherwise known as "a dc path to ground".
Capacitors block dc current and therefore there is no dc path to ground through Cin but the input bias current can flow to ground via Rin and RB keeping the op amp happy.
C2 presents as a low impedance at the node in between RA and RB, effectively creating a low impedance node in between RA and RB. This means that above a certain signal frequency, where C2 has low impedance, the input resistance "seen" by Cin will be approximately Rin (100k). At these higher frequencies there will be very little voltage ripple at the node between RA and RB. As signal frequency decreases, the impedance of C2 will increase, the input impedance that Cin "sees" will increase above the value of Rin. When the signal frequency is high enough, C2 effectively by-passes RA and RB much like the action of the emitter resistor by-pass capacitor in a common emitter amplifier. Almost all the signal amplitude appears across Rin, the 100k resistor.
An advantage of having that configuration is that C2 in conjunction with RA and RB can filter out noise on the power rail and reduce the chances of noise appearing at the op amp's positive input.