嘿 (hēi) is a casual, friendly interjection like 'hey' in English — used to call someone's attention or greet informally, and also to express surprise or admiration. It's more informal and less specific than 喂 (wèi, used for phone calls or getting attention from a distance) and 哎 (āi, often for sudden realization), and less intense than 哇 (wā, a stronger exclamation of surprise/delight).