It means that energy and mass are equivalent.
If you give energy to an object by accelerating it to high speed, then you increase its mass. The formula tells us how much the mass increases. But c is a very large number so it takes a lot of energy to have any real effect on the mass. By my estimate, if you accelerate a car to 100mph you increase its mass by about a half of 1 billionth of an ounce.
Going the other way, a small mass is equivalent to a huge amount of energy. When 1 gram of Uranium 235 splits it release a tiny proportion of its mass but that mass has energy equivalent to 1 million gallons of petrol.