As we go around the circuit, the total voltage drop must equal zero
Can someone please explain what law/principle of physics this is, and elaborate on it a bit?


It's more common sense than anything else. Kirchoff wrote it down first! Let's use an analogy:


You are on the ground floor (Floor zero in Europe) of a large building (voltage = 0 V).
You go up to the third floor in the lift / elevator. (Voltage = 3 V).
You walk across that floor to the stairs. (Voltage is still 3 V.)
You come back down to ground floor. (Voltage is zero again.)


No matter how you make the journey with any combination of stairs, lift or falling out a window the sum of the height changes will be zero when you arrive back at the start.


  My research indicates that it might be Ohm's law ...


No. This isn't directly related to Ohm's law.