Mass-energy equivalence

It means mass and energy are equivalent, with energy equal to mass times the speed of light squared. Because c squared is enormous, a small amount of mass corresponds to a huge amount of energy.

Albert Einstein introduced the idea in 1905 as part of his special relativity work. The equation E equals mc squared became one of the most famous results in physics.

In nuclear fission or fusion, the final products can have slightly less mass than the starting particles. The missing mass is released as energy, as seen in the Sun's fusion reactions.

No. It means mass can be converted into other forms of energy under the right physical process. The total mass-energy of a closed system is conserved.