What the number means
Equilibrium temperature estimates how warm a planet would be if absorbed starlight and emitted heat were balanced, without modeling a greenhouse atmosphere.
Why Earth is warmer
Earth’s measured average surface temperature is higher than its simple equilibrium estimate because greenhouse gases retain some outgoing infrared energy.
Habitability is complicated
Liquid water also depends on pressure, atmosphere, chemistry, radiation, geology, and time. A pleasant-looking number is not proof of life.