The force acting on a body moving along x-axis varies with the position of the particle as shown in the fig.The body is in stable equilibrium at

When particle moves away from the origin then at position x=x1 force is zero and at x>x1, force is positive (repulsive in nature) so particle moves further and does not return back to original position.i.e. the equilibrium is not stable.Similarly at position x=x2 force is zero and at x>x2,  force is negative (attractive in nature)So particle return back to original position i.e. the equilibrium is stable.