Under Jewish law, a “Get” is a document that a husband must give his wife to effect a divorce. Without a Get, a woman who has obtained a civil divorce is still considered married under Jewish law and may not remarry.
The biblical requirement (Deuteronomy 24:1) that the husband give his wife a Get in order to formally end their marriage under Jewish law means that a recalcitrant husband can use the Get as leverage in a civil divorce. Or he can simply be cruel and refuse to give the Get out of spite. And in Israel, where there is no distinction between civil and religious marriage, and in very orthodox communities outside of Israel, a husband who won’t give his wife a Get effectively enslaves her to him.