What is the meaning of "He'iru Varakav"?
Rashi: His radiance [will illuminate]. This is like "Lema'an Heye Lah Barak Moratah" (Yechezkel 21:15).
Radak #1: His Mishpatim will be seen to all - "Esh v'Gafris Amtir Alav v'Al Agafav" (Yechezkel 38:22).
Radak #2: The fire that He will 'rain' on them will illuminate.
Malbim: The darkness will depart and the land will be full of light from the lightning.
Why will the land be terrified?
Rashi: On that day, there will be a great noise on Eretz Yisrael, and [even] fish in the sea will shake due to it. "Va'Tachel" is like "Chil ka'Yoledah" (Yirmeyah 6:24).
Radak: All dwellers of the land will be terrified from the great, constant lightning.
Malbim: Via the light, people can see Who sent the lightning - messengers of His Hashgachah to punish His afflicters, and they will fear punishment.
