It's the brake disc. Some rust on the surface is not a big problem, but if it starts digging in to the metal mass it will cause reduced brake capacity. In Europ

It's the brake disc. Some rust on the surface is not a big problem, but if it starts digging in to the metal mass it will cause reduced brake capacity. In Europe a car with lots of rust on the brake disc will not be certified for use (mandatory checkup every two years) until the brake discs have been replaced.