A signal that warns of danger. Figuratively, it can refer to warning signs or indicators of trouble (e.g., health or economic warning signs).
車のエンジンに危険信号が点灯したので、すぐに整備工場に持って行った。
A danger signal lit on the car's engine, so I took it to the repair shop immediately.
売上の急落は会社にとって危険信号だ。
A sharp drop in sales is a warning sign for the company.
発熱や呼吸困難は重症化の危険信号なので、早めに受診してください。
Fever and breathing difficulties are danger signs of worsening condition, so consult a doctor early.