The planes x = 0, x = a, y = 0, y = a, z = 0 and z = a form a

x = 0 is the equation of YZ-coordinate plane and x = a represents a plane parallel to the plane x = 0 and X-axis is perpendicular to these planes. Distance between these two planes is a. Similarly y = 0, y = a are two parallel planes and perpendicular to Y-axis. z = 0, z = a are two parallel planes and perpendicular to Z-axis. Therefore these planes form a cube.