If you mean a diamond on the lane, that means that that lane is for BICYCLISTS ONLY, if you mean as a shape of a sign such as a "slippery when wet" sign, that usually tells you important things. I looked it up in my "Virginia Driver's Manual" and it says a diamond shaped sign is a warning sign.
Keep out: A diamond shape with a cross means boats must stay out.A buoy or sign may show only the symbol or may also indicate Swim Area, Beach, Keep Out, No Boats, Closed Area, Dam, etc.
warning sign...warning you of road hazards or upcoming objects. There is diamond type symbol found on the seals found from pre-historic Harappan Civilization in South Asia. the decrypted text shows it as a sign of currency.
In the U.S. a yellow diamond shape sign with a black S shape with an arrow.
Diamond in the US, Circle in the UK.
It is a triangular "pennant" shape, pointing to the right side. It is either yellow or orange in color.
Its a Stop sign
A white buoy or sign with an orange diamond and cross means that boats must keep out of the area. Black lettering on the buoy or sign gives the reason for the restriction, for example, SWIM AREA.
For the most part, the shape of the sign is for only one thing. And that one thing is so you can recognize what type of sign it is at a further distance away (a diamond warning sign, you will know to slow down before you read what the warning is.)
The "No Passing Zone" sign is a triangular "pennant" shape, pointing to the right side. It is either yellow or orange in color.
Orange is the sign color reserved for construction signs, like "Work Ahead", "Slow", and "Bump". among many others.
It means that you cannot pass any vehicles in that zone. It can also signify that the passing zone has ended.