Complex sentence: The grease fills gaps between the paper fibers, reducing the amount of light lost to scattering, and thereby affording a somewhat diffuse light source while blocking wind and preventing insects and other small animals from entering a structure.
Split into simpler sentences: The grease fills gaps between the paper fibers, reducing the amount of light lost to scattering. Greased paper windows afford a somewhat diffuse light source, while blocking wind and preventing insects and other small animals from entering a structure.