Here's a breakdown:
* Sub: A prefix meaning "below" or "under".
* Zero: The point at which a scale begins; the starting point.
Therefore, "sub-zero" signifies temperatures that are lower than the zero point on the scale being used.
Examples:
* "The temperature dropped to sub-zero last night."
* "This freezer can maintain sub-zero temperatures."
It's important to note that while "sub-zero" is most often associated with temperature, it can also be used figuratively to refer to anything significantly lower than a baseline or standard.