Understanding what does spatial variations of an electromagnetic waves means?

I am deriving the wave equation, in the derivation, it is considering no spatial variation in the perpendicular, in-plane y-direction and the wave is propagating along the x-axis such that:

$$epsilon = epsilon(z) $$

can anyone explain what does the sentence- no spatial variation in the perpendicular, in-plane y-direction means and signifies ??

What do you mean by spatial variations??