Iron glows in red colour when it is heated to very high temperature because:
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All objects with a temperature greater than absolute zero radiate energy in the form of light at various wavelengths. While there is technically no such thing as a perfect blackbody, nearly all objects' radiation can be modeled by the Plank function (https://en.wikipedia.org/wiki/Planck%27s_law) which tells us how much energy is radiated at a given wavelength based on the object's temperature. Essentially, the hotter an object is, the more energy it will radiate at every wavelength .
In the case of this iron bar, its temperature has increased to a point where enough of its radiation is in the visible spectrum to be seen.