They found the Rosetta Stone, which had a tri-lingual inscription that used the ancient Greek alphabet, an Ancient Egyptian alphabet (the Demotic script) and the much more ancient Egyptian hieroglyphic script. This made decoding hieroglyphics much easier and opened the door to understanding the history of ancient Egypt, using primary sources, carved by the Egyptians themselves. This gave a much fuller understanding of that culture and meant we were not reliant upon just a secondary source, like the Greek historian Manetheo's book "Aegyptiaca".