The law of conservation of energy is one of the four exact conservation laws that govern the way the universe works (the other three being linear momentum, angular momentum, and electric charge). According to this law, energy cannot be created nor can it be destroyed; however, it can transform from one form to another.
As complex as it may sound, this law is perhaps the easiest to recognize in our daily lives - and the best example of this is a ball falling from some height. If we assume there to be no resistive forces (for brevity’s sake!), the scenario will look something like this:
At the top, right when the ball is about to fall, it has maximum gravitational potential energy, and minimum kinetic energy. As it starts to fall, it simultaneously gains speed and drops in height, causing gravitational potential energy to decrease and kinetic energy to increase. This happens until the ball reaches the ground, when the kinetic energy reaches its maximum value, and gravitational potential energy its minimum.
While this explanation is pretty self-explanatory, it becomes quite interesting when you view it in the context of our conservation law. It’s not that kinetic energy is increasing and gravitational potential energy is decreasing in isolation - more accurately, we see that gravitational potential energy is transforming into kinetic energy as the ball falls to the ground!