The first law of thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another. This principle emphasizes the conservation of energy in a closed system, meaning the total energy remains constant. For example, when fuel burns in a car engine, chemical energy is converted into mechanical energy to power the vehicle. In practical terms, this law implies that any energy input into a system must equal the energy output plus any changes in internal energy. This concept is crucial in fields like physics, engineering, and chemistry, helping to understand processes such as heat transfer and work done.