تعریف و معنای "[close|shut|lock] the (stable|barn) door after the horse (has bolted|is stolen)"

What is the origin of the idiom "close the stable door after the horse has bolted" and when to use it?

The origin of the idiom "close the stable door after the horse has bolted" goes back to a time when horses were a valuable asset and were kept in stables or barns. If a horse were to escape, it would be difficult to catch, and it would be too late to try to prevent it from happening once the horse had already bolted or been stolen. Therefore, the idiom implies that it's better to take preventative measures beforehand rather than trying to fix a problem after it has already occurred.