This law of conservation of mass-energy states that neither energy, nor mass can be created or destroyed. Heat, light, sound, and electricity are different types of energy. We can convert one form of energy into another, but the total amount of energy will remain the same.
Similarly, we can transform one form of mass into another (as in chemical reaction), but we cannot create mass or destroy mass.
Since Einstein wrote his famous equation E=mc2, we have discovered that mass can be converted into energy, and vice versa. Mass and energy are really just different manifestations of the same thing. Since neither mass, nor energy can be destroyed or created, we can combine these two principles, to say that the amount of mass-energy stuff in the universe cannot change. Our universe is like a closed container, where contents of the container can change from one form into another, but the amount of stuff remains constant.
So what is an unavoidable consequence of this law? Since, mass-energy cannot create itself, it must have been initially created by Somebody outside of the container.
Sources:
"Conservation of Energy." Wikipedia. Wikimedia Foundation, 29 Oct. 2012. Web. 30 Oct. 2012. .
Lisle, Jason. Taking Back Astronomy. Green Forest, AR: Master, 2006. Print.
Digital image. Newsvine. N.p., 4 Sept. 2011. Web. .