"OldInsuranceMaps.net is a Charter Project of OpenStreetMap US that provides an open, online workflow for creating georeferenced layers of historical maps, with a special focus on making seamless mosaics from multi-page atlases in the Sanborn map collection at the Library of Congress."
What the hell does that mean? Anyone to ELI5?
This project is scanning those maps and adding the information that modern mapping software needs to show them alongside other geographic information. Where the old paper maps overlap, they work to cleanly stitch them together rather than showing the edges of the paper.
Later: The project is sort of for people looking for the old maps, so I can see why the introduction doesn't spend a life of time explaining them.
They are building computer programs that other people can use to then post online maps from old paper sources. For example, from the Sanborn map collection.
Those maps can be browsed as "overlays" on top of other maps, so you can easily compare how several maps show the same places on Earth.