What is the conservation of energy?
As an example, picture a pendulum swinging back and forth. If you don't think about air drag or friction, then during each swing the pendulum will reach the same height. This is because its gravitational potential energy, due to its height, is converted directly into kinetic energy, which is dictated by velocity.
The pendulum's total energy will be the sum of its kinetic energy and its gravitational potential energy, and this total energy will always remain the same unless it's pushed by another object
The same concepts can be applied to all closed systems.