the "m" in einstein's equation stands for mass. mass is the amount of matter there is in some body. if you knew the number of protons and neutrons in a piece of matter such as a brick, then you could calculate its total mass as the sum of the masses of all the protons and of all the neutrons. (electrons are so small that they are almost negligible.) masses pull on each other, and a very large mass such as that of the earth pulls very hard on things nearby. you would weigh much more on jupiter than on earth because jupiter is so huge. you would weigh much less on the moon because it is only about one-sixth the mass of earth. weight is related to the mass of the brick (or the person) and the mass of whatever is pulling it down on a spring scale – which may be smaller than the smallest moon in the solar system or larger than the sun.