Everything.
Generalize the idea to a volume, in other words summing up stuff over 3-dimensions (because 1-dimensional stuff is kinda boring). Imagine, for example, you want to figure out the force acting on the Earth due to the Sun. Well, you figure out the infinitesimal force caused by a infinitesimal volume of the Sun a certain distance away from the Earth. Then you do it for another piece of the Sun at another distance away from the Earth. You can do that for every little piece of the Sun. However, you want to find the whole force due to the entire Sun. Well, you integrate over the volume of the Sun and that tells you the force due to the Sun on the Earth.