In simple harmonic motion (SHM), the total energy of the system is the sum of the kinetic energy and the potential energy. At any given point in the motion, the total energy remains constant.
When the displacement from the mean position is half of the amplitude, the body is at a point where its potential energy is maximum and its kinetic energy is minimum. This occurs when the body is farthest from the equilibrium position.
At the mean position (equilibrium), the total energy is entirely potential energy, and as the body moves away from the mean position, the potential energy decreases while the kinetic energy increases. When the body reaches the point with a displacement half of the amplitude, the potential energy is zero (since it is at its maximum at the mean position) and the kinetic energy is maximum.
Therefore, at this specific point, when the displacement is half of the amplitude, the fraction of total energy that is kinetic energy is 1 (or 100%). This means that all of the energy is in the form of kinetic energy and none in potential energy.