ALUMINUM WIRING

This means you have a type of wiring, used in the late 1960s and early '70s as a cheaper substitute for copper, that is still legal for use. Though no longer used for common in house wiring at this time it is still used in many applications by utility companies such as Hydro One for service wires as the copper equivilents are very costly. If not properly maintained it could no  longer be safe.  It can have a high danger level if not properly updated.  Aluminum corrodes when in contact with copper, so connections loosen, which can lead to arcing and fires.   The solution would be to retrofit a dielectric wire nut approved for aluminum wire onto each copper/aluminum connection in all junction boxes, switches, receptacles and light fixtures. These nuts have a special grease that stops corrosion while maintaining conductivity.  Also all switches and receptacles should be replaced with componets labeled for aluminum wiring.