Here "in the shadow" represents protection or shelter. Alternate translation: "will hide in Heshbon for protection" or "will go to Heshbon for shelter" (See: [[rc://en/ta/man/translate/figs-metonymy]])
These two phrases have similar meanings and emphasize that Moab's destruction will begin and spread out from Heshbon, where King Sihon once lived. Alternate translation: because a fire will burn in Heshbon, which is the city where King Sihon lived long ago (See: [[rc://en/ta/man/translate/figs-parallelism]] and [[rc://en/ta/man/translate/figs-explicit]])
Here "forehead" and "top of the heads" represent the people of Moab as a whole. Alternate translation: "It will burn up all the people in Moab who noisily boasted very much" (See: [[rc://en/ta/man/translate/figs-synecdoche]])