The key point here is that the in is the total energy of an object - i.e. both the rest energy and the kinetic energy.
When an object is at rest, is its rest mass.
When the object gains kinetic energy, increases - thus must also increase. So when an object is moving faster, it has more mass; when it is stationary, its mass is its rest mass. But for most purposes, this increase in mass is negligible - for an increase in energy of , an object gains of mass.