What is an arcing?

Arcing is an electrical discharge across the cavity, and it should stop a microwave being used until the cause is found. The usual culprits are a charred waveguide cover, metal in the cavity (foil, a twist tie, a metal-trimmed dish), or carbonised food residue that conducts. All are cheap or free to fix. What makes arcing expensive is ignoring it: sustained arcing damages the magnetron and can start a fire.