뙤약볕 is an advanced level korean meaning a strong sun light. 쐬며 = 쐬다 + 며(하면서). 쐬다(쏘이다) is also an advanced level meaning getting something (sun light) on your body. "뙤약볕을 쐬며 자야 해" means roughtly (sorry. I can't make more proper translation with my english) "You should sleep under sun light"

해 means the sun and 빛 means light, so 햇빛 means light(s) from the sun. 살 means ray or beam of light, so 햇살 means a beam/ray of lights from the sun. 볕 or 햇볕 means the heat (energy) caused by the sun. 뙤얔볕 or 땡볕 means the blazing heat caused by the sun, especially in summer. 쐬다(contracted form) means to expose or be exposed one's face or body to light, wind, or smoke directly.