水灵灵 is a reduplicated adjective (ABB pattern) that describes something as fresh, moist, and vibrant — often used for fruits and vegetables that look juicy and crisp, or for a person's complexion that appears healthy, bright, and youthful. The reduplication intensifies the sense of liveliness and freshness. It's a colloquial, vivid word you'll hear in everyday conversation, especially about produce or someone's appearance.