Subzero means below zero, especially below the freezing point of water.
Subzero temperatures are very cold and can be dangerous.
Examples:
The temperature outside is subzero today.
In the winter, it can be subzero in some places.
Subzero temperatures can freeze water.
I don't want to go outside when it's subzero!
In Antarctica, it's always subzero.
Subzero temperatures can make your nose feel like an ice cube.