Hypoxic (low oxygen) zones mean that an area of a body of water (such as a lake or the ocean) has a very low concentration of oxygen, which makes living beings cannot survive there.
Oxygen levels can drop by eutrophication, which means that there is an excess of nutrients such as nitrogen and phosphorus, that can lead to an exponential growth of certain organisms (commonly algae and cyanobacteria) and then deplete the nutrients and die, and their decomposition consumes the oxygen in the water, which prevents other organisms of living there.
Eutrophication can occur naturally, but it can also be caused by human activities and pollution, such as the caused by fertilizers, sewage, and industrial waste.
Often, hypoxic zones (also called "dead zones" are found near human settlements.