`E = mc^2`
This is the famous energy-mass relationship equation proposed by Albert Einstein. Here, E is the energy of a body, m is its mass and c is the speed of light in vacuum. This relationship gives us the equivalence of mass and energy, i.e., mass and energy are interchangeable. In other words, given the mass of a body or physical system, we can determine the energy contained in the system. A useful application of this energy-mass equivalence is in the field of nuclear energy, where this equation can be used to determine the binding energy of the sub-atomic particles and thus the energy that can be liberated by the process of nuclear reactions (such as nuclear fission used in energy generation or nuclear weapons).
Hope this helps.