Put somewhat more plainly, we can refer to anything that stands for something else as a sign.
Gray hair is a sign of growing old.
Where there's smoke, there's fire. In other words, symbols are signs that are arbitrary and conventional. But on the other hand, because interpretation is always fuzzy due to the unique cultural and social environment in which each individual is situated, abstracting an image to its most concrete, and most recognizable or characteristic quality enables more effective communication. To form an understanding of how signs are formed, transmitted, and interpreted. This allows the designer to analyze a communication problem and to provide a coherent solution to it.