In simple harmonic motion (SHM), the total energy of the system is constant and is divided between kinetic energy and potential energy. When the displacement of an object undergoing SHM is equal to the amplitude, the object is at its maximum displacement from the equilibrium position.
At this point, all of the energy is in the form of potential energy, and the kinetic energy is zero. This occurs because the object momentarily comes to rest at the extreme points of its motion before changing direction. The potential energy is maximum when the displacement is equal to the amplitude, while the kinetic energy is minimum.
Therefore, when the displacement is equal to the amplitude in SHM, the fraction of the total energy that is kinetic is zero, and the entire energy is in the form of potential energy.