This is the 9946th most frequent Swedish word.
hettan
"The heat."
In this sentence, "hettan" is a noun used to describe "heat" specifically in the context of warm weather.
Hettan under sommaren är ofta outhärdlig.
The heat during the summer is often unbearable.
Here, "hettan" again acts as a noun referring to intense warmth that requires precaution.
Du måste skydda dig från hettan när du arbetar utomhus.
You need to protect yourself from the heat when working outdoors.
In this context, "hettan" refers to heat emanating from a specific source, namely, the fire.