In everyday life we think of "mass" as how much of something there is. How much "stuff" it is made of. More precisely in physics, an objects mass determinines two things. How difficult it is to get the object moving or change its motion (like the difference between trying to push an eighteen (thirty-eight) wheeler and a VW), and how strongly gravity acts on it (which creates its weight).
Objects can undergo chemical reactions during which they change properties, for example wood turns to smoke and ash during a fire, but the amount of each chemical element stays the same during these changes, they just combine in different ways. It is only during nuclear reactions, like those occuring in the center of the Sun, that one element turns into another (hydrogen into helium in the solar core) and in the process some of the mass is converted into energy.
Why are these two very different properties of an object (how it resists changes in its motion (inertia) and how gravity acts on it) both determined by its mass?