I found 2 nickles left at my mothers grave. What does this mean

Don't be surprised if you find more coins as ghosts like to bring coins and leave them in the strangest places. I found a considerable amount of change in the bottom of my son's locked stand up tool chest. Odd, I only pay with plastic.

And people seem to drop things from their pockets there also

It is not uncommon for visitors to leave coins at a gravesite. The origin of the custom is obscure but seems traceable into antiquity. There are many and more recent interpretations but it remains a practice among some. Here are a couple of websites that have made note of the practice and contain opinions as to the origin of the practice:

Flickr: Discussing coins on a grave in Graves, Tombs And Cemeteries

Why do people leave coins (pennies, mainly) on graves?