What is the origin of the idiom "behind bars" and when to use it?

The idiom "behind bars" originated from the physical structure of prisons, where prisoners are confined within cells that are typically equipped with bars or solid doors. The use of bars as a means of imprisonment has been prevalent throughout history, dating back to ancient times. The phrase is used to refer to individuals who have been convicted of a crime and are serving a prison sentence or awaiting trial while being detained.