Anything that has weight and shape has mass. In fact, mass is the quality that gives things weight anywhere there's gravity.
If you think about mass being about weight and shape, you can see how many of its meanings are related. If something is very big we say it has great mass — or is massive. If we talk about a large group of people, we talk about a mass demonstration or a mass of demonstrators. We refer to poor or common people as the masses — there are a lot of them. Doctors refer to tumors inside our bodies as masses.