Lower corner frequency in an emitter follower (Common collector)
For most transistor circuits the parallel combination of R1 and R2 act as a single resistor that limits the voltage on the base when frequency is low. This is because the impedance of C1 increases with lowering input frequencies. 

When the impedance of C1 matches the parallel resistance of R1 and R2 this is known as the cut off frequency or 3 dB point.