When one talks of primary "anything", they are talking about the foundation of the subject they are investigating. In this case, primary colors of light are red, blue, and yellow. They each represent a specific wavelength of visible light in the electromagnetic spectrum.
The color green is what is referred to as a "secondary color", meaning it is composed of at least two of the primary colors of light. These colors would be yellow and blue, they mix together to produce the color green, which is another specific wavelength on the visible light part of the electromagnetic spectrum.
Red and blue will mix to produce a pinkish "magenta" color as a secondary color, while red and yellow will combine to produce orange, yet another secondary wavelength on the electromagnetic spectrum.
All the primary colors combine to produce what is known as "white light" meaning it is a visible light wave combination that produces the light we see produced by the sun or light bulbs. One may take a prism and shine white light through it to break it up into it's colors that make it up. This is what happens when a rainbow is produced, the sun's light is "broken up" by the prism effect of the rain.
red green and blue are the primary colours of light. This is why your television only uses these colours.
red + blue = magenta
blue + green = cyan
red + green = yellow
The results of the above equations are the secondary colours of light.
The three primary colors are red, blue and yellow, they can be mixed together to create other colors. So green is not a primary color but it can be made by mixing blue and yellow.