Grandma currency is what you get paid with when you do something for your grandma. It is usually 5-10 times the amount of money you would get if you were doing the same amount of work for anyone but your grandma.
It takes you an hour to cut your grandma's grass. You would probably get paid about 10 bucks an hour at a landscaping company for doing that, but because it's for your grandma, she pays you in Grandma Currency so she decides to give you 100 bucks for it.
You: Grandma, I'm done cutting your grass.
Grandma: Oh, thank you so much. Here's a little something for you for doing that for me.
(Hands you a fresh 100-dollar bill)