Below zero literally means exactly what it says, below the number zero. Which shows that the number will carry a negative sign, that's a literal meaning of below zero. This can be mostly used in measuring temperature, it is a common example with the use of temperature.
However, I have another opinion. It just occurred to me that a below zero can also be below ground level, maybe under the water or an underground. But particularly, under the water. Below zero is a nice concept.
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