In a uniform electric field,

Uniform electric field means the one that is uniform and does not vary. In terms of electric field lines, it is the field in which the electric field lines are evenly spaced and are parallel to each other. In this case the equipotential surfaces are plane surfaces perpendicular to the field lines.If the line joining two points are parallel to the field lines or makes an angle $\theta (\theta \mathrm{is} \mathrm{not} \mathrm{equal} \mathrm{to} {90}^0)$, then these two points must be at different potentials.If $\theta =0^0,$the points are on the same equipotential surface .