It is a color. It is the addition of white to a color. It is the addition of black to a color. OK, those are the basics. Now let's take that knowledge and build on it.
Saturation is the intensity of a color.
Chroma is a color that is fully saturated with light. Now if a room is decorated with hues of the same saturation and chroma, that room will appear to be larger. Well, a monochromatic room will feel larger, and so will a room that has colors that are even in their intensity, such as all brights or all washed colors. Because the consistency of the color saturation lacks dimension, so the appearance of space is created. To demonstrate this, think of a room that is all white with some off-white mixed in.