The Rupee Medal is an item you can keep in your pouch that increases the amount of random Rupees dropped from enemies, by cutting grass etc.
You need a fair amount of Rupees to buy and upgrade everything in the game, so it could be useful to carry a Rupee Medal or two with you, if you’re not planning to carry much else.
That said, it’ll probably be a long time before you start amassing a worthwhile amount of extra Rupees from the medal(s).
Where to Find
There are two Rupee Medals in the game.