As its name suggest a line of equipotential is a line joining points of equal potential just like a contour line on a mountainside.
Since E = -dV/dx the closer the lines of equipotential are the greater is the field strength.
An equipotential surface is a surface which joins points of equal potential and therefore a surface on which the potential does not vary. This means that if you connected a wire between two points on the surface no current would flow between them.
The analogy with gravity could be two small lakes at equal height on a mountain side - if a pipe connected the two together no water would flow along it in either direction.