Think of it like a number. When everything is normal, temperature is at 0. Sometimes it will move from -2 (very cold) to +2 (very hot) when Months changes. All races have different temperature range they like (it's written if you over their race icon), but most will prefer neutral (0) temperature. While you can disregard that, they exhaust way quicker if they have the bad temperature.
Hot/Cold Clothes can change the perceived temperature by a girl, allowing it to bear the change of one level. You can also build furniture to directly change the temperature of the room.
Humans only like Warm-Neutral (0) Temperature therefore:
-If it's cold (-1) and you put Cold Clothes on them, all is fine.
- If it's very cold (-2) and you put Cold Clothes, they will still not be in a correct range so you can buy a Stove in the furniture that you turn On by clicking on it. The room is now only -1, and the Cold Clothes do the rest.
Undressing a girl make her bear 1 level hotter, but she will then only accept +1 temperature, while hot clothes allow her to accept 0 and +1.
I hope it's more clear!