What makes red hot so hot?

The color red is not actually hot. It is a visual perception caused by the response of certain cone cells in the eye to a specific range of electromagnetic radiation. Heat, on the other hand, is a form of energy associated with the random motion of molecules and atoms. The higher the temperature, the faster the molecules move and the more kinetic energy they have. So, while there is no such thing as "red hot", there are objects that emit electromagnetic radiation in the red portion of the spectrum, such as hot coals or a heated metal. These objects are hot because they have a high temperature and their constituent particles are moving rapidly.