In equation of a plane

ax + by + cz = d
what does the constant "d" ? is it distance from the origin to the plane ?

No, d is not the distance from the origin to the plane, that distance is |d / sqrt(a^2 + b^2 + c^2)| . d doesn't really have any significant meaning other than it's a constant associated with the equation of the plane you can multiply the plane equation by any scalar to get a different d.