Over here in America, many homeowners tend to turn on their sprinklers on for very long periods of time. Some turn their sprinklers on before going to work and leaving it on to well after they get home. This wastes a ton of water. Since grass is a plant (a very small plant), it can only absorb a small amount of water at a time or per day (2 minutes worth from a sprinkler). Because of that, the extra water will just go to waste.

So the slogan “water is dumb” is just reminding everyone that it won’t know the difference of being watered for 2 minutes or 2 hours. It will absorb all it can and waste the rest.