What is the Rosetta stone and how has it helped your understanding?

The Rosetta Stone is a stone tablet that has a King's decree inscribed on it in three languages. The three repetitions are Greek, a later Egyptian and ancient Egyptian hieroglyphics written one above another. Because it has essentially the same text written in three languages it was the key to translating ancient hieroglyphics which, until the discovery and translation of this writing, was a cryptic mystery.