Temperature affects habitability, which goes on to affect health and happiness.

The temperature displayed when viewing a planet is the average temperature of that planet. Humans prefer planets with an average temperature of 15°C (59°F). Planets such as this will have areas where the temperature reaches over 45°C, and below -15°C, but 15°C is the average for the entire planet. For every 1°C away from that perfect average temperature a planet is, the habitability of the planet is decreased by 1/6th of 1%, or 0.1667%. That means a planet with a temperature of 3°C will have a 2% penalty to habitability, as will a planet with a temperature of 27°C.

Your people can still survive at extremely high or low temperatures, but will require clinics to provide trauma centers should they ever accidentally set foot outdoors.

Terraformers will bring the temperature of the planet closer to an ideal 15°C.

-273.15°C is absolute zero. Nothing can possibly be colder than this.
0°C is the freezing point of water at sea level.
35°C is the maximum a human can survive at 100% relative humidity.
46°C is the maximum a human can survive at 50% relative humidity.
100°C is the boiling point of water at sea level.