explain what is meant by diffusion potential ,and how does it help in impulse conduction in nerves.

Resting membrane potentials often arise as "diffusion potentials" of certain ions across the membrane. The classical case is the K+ - diffusion potential.

The concentration gradient of K+ across the membrane is such that K+ tends to diffuse out of the cell without a concomitant inward flow of positive charge.  Thus, there develops a slight excess of positive charge outside the cell which, at equilibrium, opposes the further outward diffusion of K+ and accelerates the inward diffusion of Na+ and Ca2+. In a living cell, this system is in a dynamic equilibrium due to the sodium/potassium exchange pump that constantly restores the Na+ and K+ gradients that maintain the resting potential.