Silver Coins enable you to play Kung Fu Scrat and Sid's Egg Rescue without spending . (The cost without using silver coins is 5 acorns) They cannot be bought with real money; they must be earned in-game.
There are only two ways to get silver coins.
First, the Kung Fu Scrat building and the Pirate Ship each produce one silver coin every 24 hours.
Second, you can find them randomly around your village by clearing Snow Piles (or leaf piles in Dino World).
(You may see comments about a glitch involving being converted to silver coins, but that glitch has been resolved in later updates.)