How is the rosetta stone key to modern-day understanding of ancient egypt?

Rosetta Stone helped to decipher hieroglyphs, which later meant that we could read the ancient Egyptian texts: this allowed us to  understand their culture through their own texts.

The Rosetta Stone allowed the Hieroglyphs to be deciphered because it was a stone that  had the same text in Egyptian and in Greek, and Greek was known to the researchers.