嗨 (hāi) is an interjection borrowed from English 'hi' — you use it as an informal greeting, similar to 'hey' or 'hi' in English. It can also function like 'hey!' to call someone's attention. In slang contexts, 嗨 may be used as a noun meaning 'a high' (from drugs or excitement), but that sense belongs to a different part of speech and is not covered by the interjection entry.