A light came on. What does it mean?

The anti-lock brake system (ABS) detected a fault and disabled itself. Your normal brakes still work; the anti-lock function that keeps the wheels from locking (and often stability control) does not.

The supplemental restraint system found a fault and has disabled itself — in the event of an accident, the airbags may not deploy at all.

BMW's catch-all warning that the engine management found a fault and put the car into reduced-power mode to protect the drivetrain.

Audi and VW's warning that the electronic throttle and engine-management system has a fault — frequently paired with reduced power.

Depending on the make, either the same thing as the universal check engine light — an engine or emissions fault logged by the car's computer — or a routine maintenance reminder.

The tire pressure monitoring system sees at least one tire significantly under (or over) the correct pressure — or a fault in the TPMS system itself.

The traction control system (TCS) is either actively working, switched off, or disabled by a fault — the light's behavior tells you which.