InputImpedance is a measure of the apparant Impedance of an electrical load. If the Impedance is simply a resistor at the specified value, then the Impedance will be roughly constant regardless of frequency. If the load is inductive and/or capacitive, the InputImpedance will be dependant on frequency.

The InputImpedance will determine what type of driver / ImpedanceMatching circuitry is required to deliver power to the load. Most digital loads are high impedance (usually the gate of a MOSFET in CMOS) and therefore will not draw alot of power, however these devices require very small amounts of power to operate and generally don't require ImpedanceMatching. On the other hand, an aerial used for 2.4GHz wireless usually has an InputImpedance of 50Ohms, which requires ImpedanceMatching to minimise losses in transmission.