老兄 is an informal term of address used between male friends, similar to 'buddy', 'pal', or 'old chap' in English. It carries a friendly, familiar tone and is often used to get someone's attention, make a request, or express camaraderie. Occasionally it is used self-referentially (e.g. 我老兄) in a modest or self-deprecating way, though this usage is less common in modern spoken Chinese.