if we bring tip of magnet block to the center of iron block it will attract iron block but if we bring tip of iron block to the centre of magnet block it will not be attracted as gravitational force at the centre is zero,,,,,,

A conservative force is a force with the property that the work done in moving a particle between two points is independent of the path taken.Equivalently, if a particle travels in a closed loop, the net work done (the sum of the force acting along the path multiplied by the distance  travelled ) by a conservative force is zero.
gravity is an example of a conservative force, while friction  is an example of a non-conservative force.

T14855 is of trial GRE course. It is active in your account. T14858 is of Paid GRE course.