哥 literally means 'elder brother', but it's used much more broadly in modern Chinese as an informal or respectful term of address for any male, especially among friends or acquaintances (like 'bro' or 'dude'). It frequently appears in compounds like 大哥 (big brother / buddy), 小哥 (young man), and 哥们儿 (buddies). Unlike the formal 哥哥, 哥 by itself is common in casual speech and can be used to address strangers or peers in an approachable way.