It indicates that something is banned or forbidden.
This sign is effective across cultures largely because it symbolizes a universal negative. The red circle with a line through it, often called a "no symbol," is intuitively understood to denote prohibition, such as in "no smoking" or "no entry" signs. It requires no knowledge of a specific language and relies on visual cues that signify prohibition, which makes it understandable regardless of an individual's language or cultural background. Therefore, the sign works well even when people do not speak the same language. It's also frequently red, which is a color that naturally draws attention, and its design is simple enough to be adapted for various purposes by changing the image or text within or below the circle.