Energy cannot be created or destroyed. It can only be transferred from one store to another, or dissipated, which means spread out into the surroundings so that it is stored in less useful ways. This is the law of conservation of energy.
A closed system is one where no energy enters or leaves. Inside a closed system energy moves between stores, but the total amount of energy stays the same. There is no net change in the total energy.
Think of a toy with a wound-up spring in a closed system. At the start, 20 J is in the elastic store of the spring. As the toy moves, energy is transferred to its kinetic store, and some is dissipated as thermal energy by friction and as sound. When the toy stops, the useful energy has gone, but the total is still 20 J. It has simply ended up spread through the surroundings, where it is no use to us.