Analogy: Say you have a sheet of paper. You stack another piece of paper on top of that sheet. You continue doing this until you have a very thick, voluminous rectangular prism composed of many sheets of paper.
In the same way, in volume using cross sections, we sum up the prisms that make up the solid.
This can be hard to visualize, so use this website: https://www.geogebra.org/m/XFgMaKTy
Volumes of Solids with Known Cross Sections (cr: Calculus BC)