What is energy conservation?

Physics is full of "conservation" laws. There is a law of conservation of mass and a law for the conservation of momentum. The law of conservation of energy works the same way. In simple terms, the law of conservation of energy states that energy cannot be created or destroyed. To be a bit more specific, the law of conservation of energy is referring to the energy within a "closed system." In reality, a closed system is a bit left up to the person doing the calculations. The closed system is the group of objects that are interacting with one another and exchanging energy. In other words, the energy of a closed system will never be lost. It can change from energy type to energy type, but the total energy being exchanged remains the same.
I like to use a roller coaster when I teach this concept to students. Let's simplify and say that the closed system that is the roller coaster is the track, the cars (with wheels), and the air immediately surrounding the roller coaster. When the roller coaster is at the top of the tallest hill, the coaster's potential energy is at its maximum. That potential energy will then be converted to kinetic energy as the coaster goes down the hill. Ideally, all of the potential energy of the hill is turned into kinetic energy of speed. If the hill gave the coaster 1,000 J of potential energy, the coaster should have 1,000 J of kinetic energy at the bottom of the hill. The total energy of the system was conserved. None of the energy was lost, and the coaster didn't end with more energy than it started with.
Some savvy students like to point out that the coaster's 2nd hill should be able to be as tall as the first hill if all of the energy was conserved and converted to kinetic energy of moving a set of coaster cars. Unfortunately, that isn't possible. The reason for that is because not all of the initial potential energy was converted into "useful" energy. Friction exists within the system as does some vibration. The friction causes a conversion of energy from potential to thermal energy. The thermal seems to be "created," but it is result of an energy conversion. Vibrations will cause sound, and sound energy is a result. The thermal energy and sound energy are not useful forms of energy for roller coaster speedy fun, but the energy does get included into the closed system. The initial potential energy might be 1,000 J, and some of that will be used to create sound, heat, and coaster speed. It's helpful to write it out as an equation to see the conservation. It might look something like the following equation.
1,000 J potential energy = 850 J kinetic energy + 100 J thermal energy + 50 J sound energy
http://www.nyu.edu/classes/tuckerman/adv.chem/lectures/lecture_2/node4.html

https://energyeducation.ca/encyclopedia/Law_of_conservation_of_energy