What does imperivm neptuni regis mean on the shellback certifcate?

"Imperivm Neptuni Regis" translates to "The Empire of King Neptune" in Latin. On a shellback certificate, it signifies the initiation into a maritime tradition where sailors are recognized for crossing the equator, often marked by a ceremonial event involving the mythological figure of King Neptune. This phrase emphasizes the sailor's newfound status and connection to the maritime lore associated with Neptune.