This expression is used to describe something that seemed to have great promise but doesn’t “pan out” (prove) to be of lasting value. It apparently has two legitimate origins. Wiktionary says it came from the small charge of powder in the firing pan of a flintlock rife igniting but failing to ignite the powder that fired the ball. There was an impressive flash, but it was ineffectual. The Phrase Finder writes that the expression was also used by California Gold Rush prospectors who “…became excited when they saw something glint in the pan, only to have their hopes dashed when it proved not to be gold but a mere ‘flash in the pan’.”