The electric potential, which I will denote by $\Phi$, is
originally defined by the following relation to the electric
field (if the math is unfamiliar, don't worry I'm just including
it for completeness) \mathbf E = -\nabla \Phi One consequence of
this is that The electric potential is only defined up to an
additive constant This means, in particular, that one has the
freedom to pick any point in space, usually called a reference
point, at which the potential is zero. Once you pick this point,
then the value of the potential $\Phi$ at any other point is
completely determined by the definition above. However, if you
don't choose such a point, then additive ambiguity in the
definition of potential makes it so only calculating differences
in potential makes sense. In this case, it wouldn't make sense
to say that "so and so point in the circuit has such and such
value." Punchline. The electric potential is defined in such a
way that only differences in potential make sense unless one
picks a reference point at w