What Einstein meant by that equation was the ratio between Energy and Mass is always constant, which is the square of speed of light, c.
He also imply that, for any particle, as its speed or velocity increases, energy also increases and its mass also increases. Well, one maybe don't understand this, but think of a stone thrown at you. Let say that the stones are thrown at equal speed, but different sizes, from small to large. You might die immediately if the large stone is thrown at you. The larger the object, the more energy it carries.
Certain particles has constant mass but of excessive energy, therefore the ratio exceeds the squared of the speed of light. How will this possible? is there any other particle faster than light? Definitely. Einstein postulated that speed of light is not constant but finite. Some heavy particle may travel 98% the speed of light. Some particle maybe 102% the speed of light.