The color wheel is a visual representation of colors arranged by their color values. Understanding the color wheel provides artists with important fundamental knowledge for proper pigment selection and color correction. The color wheel shows the relationships between primary colors, secondary colors and tertiary colors.
They are considered primary colors because they cannot be made by mixing other colors together. These three colors are the base for every other color on the color wheel which is why they are considered primary.
When you mix primary colors together you get secondary colors. On the color wheel, they are located between the primary colors.