What is the origin of the idiom "ace in the hole" and when to use it?

The idiom "ace in the hole" originates from the game of poker, where players are dealt a hand of cards, and one card (the "hole card") is dealt face-down and kept secret until the end of the game. The ace is typically the highest-ranking card in many variations of poker. Therefore, this idiomatic phrase is used to describe a secret advantage, resource, or strategy that someone keeps hidden and reserves for use in a pivotal or unforeseen circumstance.