Literal traffic signal meaning: the red light that tells vehicles or pedestrians to stop.
Please stop at the red light.
red light; stop signal; danger sign
Figurative use: a warning sign that something is wrong, risky, or should be stopped. Often used in business, health, or project contexts.
This project is already a red light (in trouble).
My health checkup results showed a red flag.
The car stopped at the red light.
He ran the red light and drove off.
A red flag went up over the company's management.
A red flag went up regarding his health.