Let's ignore the terms voltage and volts for now and get an understanding first.
Imagine I had a frozen chicken of mass 1kg hurtling down from 1km above the earth's surface. For now we can approximate g=9.81 and get to work. At its initial height of 1km, the chicken has a potential energy of mgh=9810J. In other words, as the chicken plummets from the sky, it will gain a kinetic energy of 9810J (engineers do not know of air resistance). Now lets say we had a lot of chickens we wanted to drop from the sky, we need a better unit.
So I bring to you, the Poult. The Poult (P) is the energy gained by a chicken as it falls from one point to another. This makes it simple, now the point 1km above the ground is 9810P higher than the surface of the earth. Any chicken falling through this height will gain energy of 9810J. We can say that the poultage of the point 1km above the surface is 9810P.
So now circuits. It's basically exactly the same thing. The volts are the amount of energy a coulomb worth of electrons will release as they travel from one point to another. We can say that the volts at a certain point are 0V just as we can say that the poults on the ground are 0P. It may not scientifically be 100% correct, but as engineers, that's not a concern. Instead setting the surface to 0P gives us a convenient way to define the poultage at all the points above the earth. Same for circuits.
Hope this helps.