Refers to the head of a group (組). In organized crime contexts, it specifically means a yakuza clan boss. In ordinary workplaces, it can mean a foreman or team leader, though 班長 or リーダー may be more common in some settings.
He is a yakuza boss.
工事現場の組長が作業を指示した。
The foreman at the construction site gave instructions.
He became the boss of that group at a young age.
He succeeded his father as the boss of the clan.