What Does "jal ja" (잘 자) Mean in Korean?

This is a very casual and affectionate phrase. It's used primarily between people who are very close, such as romantic partners, close friends, or family members (especially older siblings to younger ones, or parents to children). It would sound inappropriate or even rude if said to an elder, a superior, or someone you don't know well. It conveys warmth and care, often used as a final send-off before going to bed. It’s the kind of line you’d hear between friends sharing a room or a couple ending their day.