I don't know if you mean red-green colorblind or all colors. I'll try to explain.
They are the opposite colors in the color circle. Or just the opposite colors. The primary colors is red, blue and yellow. These are colors you can't make with other colors. The basics. If you mix equal parts yellow+blue you have green, yellow+red is orange and red+blue is purple. If you put these in a circle, starting with red you'll have red, orange, yellow, green, blue, purple. Like this:
In the circle, it is the opposite colors. Aka, red and green, orange and blue, yellow and purple. This is like black and white, which is total opposites. All colors have a complementary. In a way they have the opposite value. I hope this helped.