A 'glow-up' is a dramatic, positive transformation — usually in appearance (skin clearing, fitness, confidence) but sometimes in life circumstances too. Popularised by Chief Keef's 2013 song 'Gotta Glo Up One Day'. If your teen talks about someone's glow-up, they are admiring their transformation. Harmless and usually celebratory. The opposite is 'glow-down', which describes a decline.