The microwave safe symbol is simply an image of a microwave. The product also might have the words microwave safe written on it.
All plastic container have standards set by the United States Food and Drug Administration that they must meet in order to be labeled as microwave safe. Plastics that do not meet these standards could possibly release toxins into the food when heated. The container will be labeled with a symbol or written instructions with uses it´s safe for. If a container is not labeled or the person is unsure how to heat with that container, glass may be a better option to use when reheating.