Supplì are what we might call “rice balls”, an unromantic name for this antipasto or street food that has two pockets of distribution: Rome and the island of Sicily, where they are called arancini for their color and size.
Supplì have a long history, but the current Roman version starts with cooked rice (in households this is often leftover risotto) with a bit of tomato sauce wrapped in a ball or egg shape around a piece of mozzarella. The ball is then drenched in egg and covered in bread crumbs before being deep fried. The Sicilian arancini may include meat sauce (ragù) and peas. Shops may sell suppli in many versions with different stuffings.
When you break a Roman Supplì in two and separate them, the warm mozzarella will make thin strings between the two halves. This gives the concoction the nickname supplì di telephono, recalling telephone wires. This usage is going out of fashion as the telephone wires disappear from the landscape.