The document discusses the energy of a horizontal mass-spring system. It is the sum of the kinetic energy (1/2mv^2) and the elastic potential energy (1/2kx^2). It provides two scenarios: (1) when the spring is expanded, the displacement is positive, and (2) when the spring is compressed, the displacement is negative. In both scenarios, when the mass is released, it accelerates back towards its initial position due to the elastic restoring force of the spring.