The reference is to planets being Via Combusta, in the Fiery Road or "Burned Path", which consideration goes back to Greco/Roman times (but is not found in Vedic astrology, nor in Modernist astrology) The Via Combusta (15 Libra to 15 Scorpio) doctrine indicates a debilitation for planets posited therein, excepting for Saturn and Mars (most of the ancients exempted these planets from debilitation due to the exaltation of Saturn, the terms of Saturn and Mars, and the rulership of Mars-in the Scorpio part of the road) Moon, Venus, Jupiter, Mercury and Sun, were (are) considered most especially detrimented when "in the Road".
My own opinion is that there is some degree of validity to this ancient doctrine, but that these detrimental Via Comusta influences are more pertinent to elections, event, horary and mundane delineations, than to natal, although I do believe planets progressing or transiting through the "Fiery Road" can be somewhat modified in their progressed/transit influences upon the native.