I'm in a group chat with some Italian people and they always say "uao". I already search a lot on Google but wasn't able to find anything about it. Does someone can tell me what that means?
It's an Italianized spelling of "wow", very likely. From this online dictionary:
uàu (o uào) interiez. – Nel linguaggio dei fumetti, adattamento fonetico e grafico della voce onomatopeica angloamer. wow (v.).
By the way, I suggest that you always include some context (one or two sentences) in your questions about a translation; it's often difficult to understand a single word with certainty without context!