It is when you set your CPU voltage to something in BIOS, and then you read the voltage in CPU-Z, or something similar, and the voltage is lower. For example: You set the voltage to 1.5V and you read it as 1.4V in CPU-Z.
I think that vdroop is supposely a safety measure by motherboards to protect the cpu, Vdroop is used to cut costs - basically, they can use less capacitance on the output of the voltage regulator and still supply "good enough" power to the CPU by simply overvolting it slightly when it's not drawing much current.
vDroop - the difference between the idle and load voltage.. Generally, you get a higher voltage when idling than when at full load. That's because it works as a self-protection mechanism to prevent too much voltage surging into the chip and frying it, or not giving it enough power.
For example, if your chip needs can take a voltage range of 1.35v - 1.55v, you might find that your idle voltage is around 1.50v. When you put it under full load, it'd drop to 1.47v. That's vDroop.
The board gives it extra voltage at idle so that when the chip goes into full load it can handle the negative voltage drop without going below the minimum 1.35v.
Likewise, the voltage upshoot from from full load to idle is protected by vOffset. That's the difference between your idle voltage and the maximum the chip can handle. You might find that when switching from full load to idle the voltage goes over the 1.50v and then comes back down to settle at 1.50v. That's to give it some room up top away from the 1.55v maximum.