'Cringe' means painfully embarrassing — content or behaviour so awkward you physically wince watching it. 'That's so cringe' = 'that's painfully awkward'. Originally a verb (to cringe = to recoil), now widely used as an adjective. If your teen calls something cringe, they are passing aesthetic judgement. Harmless. Among Gen Z, calling something 'cringe' itself is sometimes considered cringe — 'cringe is the new sincere' is a real meme.