Coins are Pet Rescue Saga's main currency. They are, however, only used for one thing, primarily, and that is to purchase the Block Buster Boosters
. To earn Coins, you simply need to progress through the game, completing each stage.
Many players will find that they accumulate far more coins than they are able to spend on Block Buster Boosters
. Despite this, there is in fact no other resource to spend your Coins on. It's worth saving them up but, all the same, they are nowhere near as important in Pet Rescue Saga as they are in other Facebook games. If you want an easy route to succeed, you're going to need Facebook Credits.