In Your Tea Cup
Color saturation is one of those terms that sounds terribly technical. You just know that to understand it will take at least a Master's Degree in Color Theory.
Actually, the opposite is true. Understanding color saturation is as easy as brewing yourself a cup of tea.
Drop a tea bag into boiling water and what happens? As the tea steeps the water becomes more and more saturated with the color of the tea. The color being release into the water by the tea is always the same. When there is less of it in the clear water it looks light. When there is more of it, when it is more saturated, it looks dark.
We can go in the other direction and dilute cranberry juice with water. The more water we add, the more diluted the cranberry juice is, and the more unsaturated the color becomes. It's still the same red, but there is less of it, so it looks lighter.
This time we're going to dilute the tea with milk. The more milk we add the less saturated the brown of the tea becomes.
However, diluting with milk is different than diluting with water. Water is clear, but milk is white. The result is that with milk we not only reduce the saturation of the color but create a tint of it at the same time.
We could reduce the saturation of the tea in other ways. We could, for example, add black and create shades. However, the only black liquid in the house is ink, and I'm not about to drink it, nor will my wife let me. So you'll have to imagine the results for yourself.
As a quilter you have to decide how bold a statement you want to make with your colors. The more saturated colors are, the bolder they are. You need to keep saturation in mind when choosing fabrics.