used to refer to a situation in which there is a choice between two different plans of action, but both together are not possible

It's an either-or situation - we can buy a new car this year or we can go on holiday, but we can't do both. 这就叫鱼和熊掌不可兼得——今年我们要么买辆新车，要么去度假，但只能做一样。