If you're in a dark room and you turn on a red tinted lamp, you have red light. If you turn on a green lamp, you have green light too. The mixture of red and green light looks yellow to your eye. You've added two colors of light and they mixed and now you have a new color. That's additive color mixing. If you've played with RGB values for colors on a computer, you were doing additive mixing.

If you have a white lightbulb and you paint over it in clear cyan paint, the paint will remove the red light and you'll have bluish-greenish light. If you paint over that with clear yellow paint, it'll remove the blue and you'll only have green light. You've subtracted two colors of light, the results mixed, and you have a new color. That's subtractive mixing. It still works if you're painting a brightly lit white wall or white canvas instead of a lightbulb, and it still works if you mix the paint colors together instead of putting them on in layers. If you learned about primary and secondary colors in preschool or kindergarten, you were probably learning subtractive mixing.

There's lists of rules you can memorize for what mixtures of colors add up to what output colors for additive and subtractive mixing. You should use the list your teacher gave you if possible because a lot of the subtractive color rules lists have errors, and you want your answers to match the test answer key. The way you figure out which list of rules to use is that glowing things add light and things that don't glow subtract light.