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

At both $x=x_1\text{ and }x=x_2$ is zero but at $x=x_1$ if displacement is positive the force is also positive. This is unstable equilibrium

at $x=x_2$  if displacement is negative, force is positive hence , it is at stable equilibrium