A "sign" is a physical message. It can be written or formed with the hands. It can be composed of words or symbols.
A "signal" is the message intended to be conveyed by the sign.
For example, a "stop sign" is a posted image with the word "STOP" printed on it, or it can have a picture of a raised hand printed on it. The signal it conveys is "stop your car!".
The two ideas are often confused, so there is much overlap.
It is similar to the difference between "numbers" and "numerals". A numeral, like "3", is a symbol. The idea it conveys, "three", is an idea that exists apart from the numeral.