It is a mathematical consequence of Einsteins theory of special relativity and of his formula E=mc2. It says that energy and mass is basically the same thing. You may think of mass as “frozen” energy. Mass can turn into energy and energy into mass.

When something that has mass moves, the energy that is associated with that motion will add to the mass. As the object approaches the speed of light, its mass approaches infinity. This mass used to be referred to as “relativistic mass” but the term is out of fashion. This increase in mass actually does not mean that something strange is happening to the object. It has more to do with the geometric properties of space-time.

So, unless you are a physicist, just think of it a bizarre effect of relativity!

For more “relativity special effects” you can read George Gamow’s “The New World of Mr Tompkins”.