As you've correctly stated, this law dictates that energy cannot be created or destroyed; it can only be converted from one form to another. For instance, chemical energy can be converted to kinetic energy in the human body. In an isolated system, the total energy is constant.

This law says that the total entropy, or randomness/disorder, of an isolated system can never decrease over time. It also states that energy transformations are not 100% efficient, and that some energy is always lost as waste heat. This is why perpetual motion machines are impossible - they violate the second law of thermodynamics.