The scan above may not show it on some monitors.
This one should be a bit clearer. One of the Cobb bat on cards.
The plates have a surface that retains water and the image is put on with a medium that repels water but would hold oil. When the surface gets scratched the scratch can hold some ink and will usually print.
You can actually make a lithograph using a piece of paper,crayon, a roller and some oily ink. Not a good or detailed lithograph, but something.
Steve B