Definition & Meaning of "the penny drop "in English

What is the origin of the idiom "the penny drops" and when to use it?

The idiom "the penny drops" is believed to have originated from the concept of old mechanical slot machines or coin-operated devices. In such machines, a penny or coin would physically drop into a slot or mechanism to activate or unlock a particular outcome. The phrase represents the moment when the coin drops and triggers a realization or understanding in a person's mind. It is commonly used when someone finally comprehends a concept, recognizes the truth about a situation, or perceives the implications of a statement or action.