A symbol that features a circle with a diagonal slash through it is indicating that something is not allowed. The "no symbol" is seen in many different places and on many different things every day.
Government buildings, such as the capitol building, use the "no symbol" to indicate that visitors are not allowed to bring weapons, drugs, liquids and many other items into the building. Restaurants use the symbol with an image of a burning cigarette in the background it to indicate that smoking is not permitted on the premises. Similarly, electronics manufacturers use the symbol with an image of water droplets in the background to indicate that the item should not be used near water.