It means below freezing level.
the subfreezing temperatures sent chills down my spine.
sub
I am wearing my parka because the weather in Toronto has reached subfreezing temperatures this week.
the hurricane led to a subzero temperature in ohio
It forms sleet.
The middle syllable "free" is stressed (sub-FREE-zing).
Frostbite
Sleet
frost Bite
Salt is often put on the sidewalk during or after it snows. This is because salt lowers the freezing point of ice. This is often just enough to melt snow at subfreezing temperatures.
The humidity in winter is lower.