哥们儿 is a colloquial term for a close male friend, equivalent to 'buddy', 'bro', or 'homie' in English. It's heavily used in Northern Mandarin, especially in casual conversation among younger people. The 儿 (ér) suffix marks it as colloquial and affectionate; the word is often written as 哥们 in less informal writing. Use it between friends, not with strangers or in formal settings.