Saturation means holding as much moisture as possible. When you water your houseplants, you may soak them until the soil around each plant reaches saturation.
The noun saturation means the act of completely soaking something until it's absorbed as much water as it can. Imagine a sponge that can't get any more wet, and you'll have an idea of what saturation means. When you're talking about art, saturation means the amount of bright, undiluted color in a painting or photograph, usually colors that are quite intense and unshaded. The Latin root saturatus means "to fill, sate, or drench."