It is written (Psalm 84:12), "For a sun and a shield is Havayah Elokim (G-d, our L-rd)." "Shield" refers specifically to that shield which is a covering for the sun, to protect creatures so that they should be able to bear it (i.e., the sun's heat).
Our Sages, of blessed memory, have said (Nedarim 8b), "in the Time to Come (i.e., in the Messianic Era), the Holy One, blessed be He, will take out the sun from its sheath; the wicked will be punished by it...," as they will be unable to bear the intensity of the sun. The passage goes on to say that the righteous will not only be able to tolerate it, they will actually be healed by it.
The Alter Rebbe, Shaar Hayichud Vehaemunah, Ch.4; Moshiach Now! & From our Sages, reprinted from LchaimWeekly.org - LYO / NYC
