This is a pretty common expression in Korean although I only get to know what it means a little late while I was talking to a Korean friend. I was so upset then and he said to me this phrase — 힘내요 (himnaeyo).
It means cheer up! It also means the same as the cliche ‘break a leg’ in English. When you want to uplift someone’s spirit or give encouragement, this is a good expression to say. To someone esteemed, one can say the more polite way 힘내세요 (himnaeseyo). This is in addition to common expression like 화이팅 (fighting) or 아자 (aja).
*image was taken from firemasterxd.devianart