Hot objects emit radiation. If the radiation were visible, say visible light, what would be the temperature of the body emitting the radiation? For example, for soft white light, it is accepted that the body must be at about 3000K. This 3000K is the colour temperature.
The usefulness of this knowledge in photography? Immense, unless you want to spend hours in the darkrooms making the photos look natural. The two photos below shall explain better.
This one is a no colour correction photo, while the one below is with proper colour temperature settings dialled in.
Notice how the colour is realistic now. Since the place was lit by dim yellow lights, everything had a yellow tone, which is not the natural colour.
The settings that need to be tweaked within the camera to get realistic colours is the white balance. Dial in the on which matches your setting best, and shoot away.
Last time, it was Composition.