I've seen this signboard in use several times, in several places.
It is placed by those engaged in cleaning a place to warn others of the hazards of a wet floor, or wires running across or work in progress. One way of looking at it is health safety. That one does not hurt oneself in such a situation - by accident. The other way of looking at it is exhorting patience. The one at work asks for time to do his or her work and that you do not intrude upon or cause him or her to do double work of cleaning.