According to the Law of Conservation of Mechanical Energy, in absense of external forces, the mechanical energy of a system remains constant.
Considering the Earth-person system, when the person slides down, its potential energy is reduced (since potential energy is proportional to the height at which an object is placed).
Since potential energy is reduced, kinetic energy must increase to keep the total mechanical energy constant.
Therefore, the potential energy is transformed to kinetic energy as the person slides down, which is noticeable because the speed of the person will increase.
