Imagine you have three special colored crayons: red, blue, and yellow. These are called primary colors. You cannot make these colors by mixing any other colors together. When you mix these colors together, you can make all the colors in the whole wide world!
For example, if you mix red and blue together you get purple. If you mix blue and yellow, you get green. And if you mix yellow and red, you get orange.
But no matter how many colors you mix, you can never make a primary color. They are special and unique, and they are the start of all the other colors we see.