Chips指(作赌注用的)圆形筹码。The chips are down来自扑克牌桌，描绘的是下赌注的筹码都已经放在桌面上而打扑克的人即将摊牌一决输赢的当头。这是最让人提心吊胆的时刻，因为结果未见分晓。后来人们把这个习惯用语广泛地应用在其它场合。

when you are in a very difficult or dangerous situation, especially one that makes you understand the true value of people or things

在困难时刻，在危急关头

One day when the chips are down, you will know who your true friends are.

等到危急关头你就会知道谁是真正的朋友了。