Turbidity is a quality of cloudiness or murkiness, particularly of water. The turbidity of the lake at your summer camp might make you wonder if the water is safe for swimming (of course, if you stand there wondering for too long, your bunkmate might push you in).
When the turbidity of a liquid is high, that means it's hard to see light through it — it's nearly opaque. A body of water's turbidity can reflect how healthy it might be for organisms living there. For example, the turbidity of your fish tank can tell you a lot about how recently it's been cleaned. The Latin source of this word, turbidus, means "muddy or full of confusion."