Well, imagine you're taking a bath with your rubber duckys and you're so busy playing that you forget to add fresh water from time to time. After a while, the water gets all yucky and smelly. This is what happens in some parts of the ocean.
Anoxic waters are water bodies that have very little or no oxygen in them. This happens when there's too much algae or other tiny plants and animals in the water. These creatures need oxygen to survive and reproduce, so when they are too many, they use up all the oxygen.
Just like when you ran out of fresh water in your bathtub, the water in the ocean is not able to support many other living things that need oxygen to survive. These areas can become "dead zones," meaning that no life can exist there, just like when you need to drain the water in your bathtub and start fresh.
A lot of factors can contribute to the formation of anoxic waters, like agricultural runoff, pollution, or climate change. Scientists are working to find ways to prevent the formation of these dead zones to protect marine life and the health of our oceans.