The short and long of it is the substance; the plain truth. It is used to refer to something which is unambiguous and may be described quite simply – the long version and the short version being the same. For example, “You can debate the 1971 Ali/Frazier fight all you like but the long and short of it is ‘Frazier won'”.
For some reason, later users of the expression have preferred to use the reverse form – ‘the long and short of it’.