In plain English
Lights are ahead — and they might be on red when you arrive. Ease off and be ready to stop, especially if you can't see the lights yet.
Where you'll see it
Before lights that are hidden by a bend or brow of a hill, and before the first set of lights after a long stretch without any.
The common trap
Assuming the lights will be green because the road looks clear. The warning exists precisely because drivers arrive too fast.
Memory anchor
A tiny traffic light inside a triangle: the road saying "lights ahead — guess red, not green."