Peperoncino refers to several thin-walled, medium sized hot peppers used often in Italian cooking.   They are grown in the Calabria region of southern Italy, and the pepper is fruitier and spicier than a typical red chili. 

The peperoncino flakes are easy to find and keep well in the spice rack and are added to certain Italian dishes to add heat and flavor.  You can also find the powdered versions in some specialty shops which tend to be quite spicy.  In Calabria, it is typical to add this ingredient to many of their local dishes.