تعریف و معنای "when the chips [are] down"

What is the origin of the idiom "when the chips are down" and when to use it?

The idiom "when the chips are down" originates from the world of gambling, where "chips" represent one's bets or stakes. In this context, it is used to refer to a situation in which one is facing a challenging or difficult situation, often when one's circumstances are at their most critical.