汗流浃背 is a four-character idiom (成语) meaning 'drenched in sweat; streaming with sweat from head to back.' It is used to describe someone sweating profusely due to extreme heat, intense physical exertion, or nervousness / shame. The image is sweat flowing so heavily that it soaks the back of the clothing. Unlike the literal translation 'stream with sweat,' the idiom carries a stronger sense of being completely soaked. You can use it in both literal and figurative contexts (e.g., from stress).