The Voltage drop should be anywhere from 3.6 to 4.1 depending on the specific LED (I've seen .3 volts in variation) and the target current. In series, add the individual drops and in parallel, the foltage drop will be the same but the current will be 4X.
If you are asking what voltage drop itself is, in layman's terms (in my case, lame-man's terms), voltage drop is the voltage measured across a component in a circuit. I believe it is more specific to a series circuit where the total voltage supplied is shared in an additive way by all the components; the sum of the voltage drops across all elements is equal to the supply voltage (Vin).
Perhaps your question can be answered in a more enlightened manner by one of the more knowing members here. And I won't mind being corrected as long as I can remain seated; I'm tired of standing