Impedance vs resistance
Impedance is resistance + reactance. Resistance is the opposition to flow of current, and entails power loss, usually in the form of heat.
Reactance is the opposition to alternating current flow due to inductance or capacitance, and does not entail power loss. Capacitors and inductors store energy for a certain amount of time, and thus do not dissipate any power.
Formula
The formula for a capacitor’s impedance is
Where f is the frequency, and C is the capacitance.
Since the impedance depends upon the frequency of the alternating current, we observe that the capacitor is a frequency dependent resistor. This gives rise to the capacitor’s utility in attenuating certain signal frequencies.
Example
We must create a filter in order to attenuate signals, such as a low-pass filter.
As we can see from the formula above, the impedance of the capacitor decreases as the frequency increases. When this happens, the capacitor will let more current through to ground, and the output signal’s current will decrease. It effectively steers high-frequency current away from the signal output.
Electro Spirit 
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