There are certain integrals which say integration over all m ,integration over all area,integration over line. I am confused with this. In calulus i am comfortable with integration with limits of an independent variable and the integration results in the area under the curve. But in doing the moment of inertia of a solid about the axis a small dm is present inside the integral and we say to integrate it for M. What is this integration about M exactly . Similarly what is integration over line and area. How can i visualize this?