Scarpetta means slipper, but fare la scarpetta doesn’t have anything to do with making slippers. It means to wipe your plate clean with a piece of bread, something all my Italian friends do on informal occasions.
There are several opinions about its origin. It seems to come from southern Italy. Perhaps it’s a metaphor likening a shoe scraping along the ground picking up whatever it finds to the crust of bread mopping up the sauce in the plate. Or maybe it refers to ‘scarsetta‘ or poverty which obliges people to content themselves with whatever there is, usually very little.
A third opinion suggests that the fingers pushing the bread around cleaning up the plate looks like a shoe with a leg coming out above. Take your choice.