The journey across the River Styx is the journey from life to the afterlife.
The journey is accomplished in a ferry or boat.
The ferryman, or boatman, has to be paid.
The basic coin of that era was an "obol"; plural "oboloi": not gold; not silver; very basic. You might translate this as "penny".
The fare was two oboloi.
So, as part of the funerary ritual, someone - a loved one, rellie, friend - would put a penny on each eye, by way of preparation for the soul's journey.
"Hades" in Greek mythology is not the negative concept of "Hell".
Or sometimes a coin would be placed on his mouth, under the tongue. They thought the fare was only one coin.