Bacterial blooms are a sudden increase in the bacteria that lives in your fish tank. The water will all of a sudden become very cloudy. This usually happens when you first set up a new tank or introduce new fish to an existing tank.
Once your tank begins to "cycle" a bacterial bloom will take place. This is exactly what it sounds like: a large colony of new bacteria is alive and working to consume the ammonia in your tank. The beneficial bacteria is what causes the bacterial bloom and can cause the water to look very cloudy.