An indicator light or illuminated signal, usually colored red, yellow, or orange, on a vehicle dashboard intended to convey a condition of which the driver should be aware. Most frequently, it indicates a system malfunction or a potentially dangerous condition (e.g., low oil pressure, high coolant temperature). It may be a word or words ("Charge"; "Check Engine") or a symbol which may or may not be readily identifiable.
The derisive term comes from the fact that such lights typically take the place of a gauge, meter or other indicator that can provide more substantive and detailed information. The binary (on/off) nature of the light is considered inferior to an indicator which provides data on the condition, but which requires some interpretive thought by the operator.)
"Get the manual out of the glove box so I can look-up what that idiot light that just came on means."