2 Answers

Hi,
This is actually an equation showing relation in between Potential difference(V), Intensity of Electric Field(E) and the distance between two parallel conducting plates.
Consider a system of two conducting plates with one plate having positive charge and the second plates induces negative charge in it.Hence the direction of Electric field intensity inside the plates will be shown from positive plate to negative plate.
Suppose the distance between two plates be given by "d" and the Electric field intensity be "E".
As we know the expression of Electric field is given as
E=(k *q)/d^2.........(1)
Here
K is Dielectric constant of the medium between two plates
q is charge accumulated on the plates
d is distance between two conducting plates
Now,the electric potential is given as
V=(k*q)/d............(2)
Putting this value in eq(1),we get
E=V/d
or V=Ed
Hope you got that!!!
Source(s): Theory of Electrostatics 
This is a simplification of the integral calculation of the electric potential (voltage). Usually:
V = integral( E dot dl )
where "E" is the electric field, and "dl" is a differential length along the path of calculation, and "dot" signifies the vector dot product.
If the electric field is a constant, and the path traveled is a straight line of length "d", aligned with E, the integration simplifies to multiplication:
V = E*d