It uses a color wheel to visualize the relationship between colors and simplifies the process of creating a harmonious or contrasting color effect for your designs. It is a set of suggested guidelines and rules for creating color combinations or visual effects of specific color schemes.
While that sentiment holds some truth, color theory is both the practical science and creative art of using color. Should not art be based on creativity and not theories? You might hear it tossed around and wonder what it means when you are talking to artists and designers.