What is the function of credit on facebook?

It should say where the 20 credit is good at near the credit. I have one for Restaurant City and its a game I play on Facebook. If your planning to buy something for the game you play, then you can use the credits. I haven't used them.

Credit is the currency used on Facebook By Credit FB, you can buy bidding or using a special feature on the application on FB. For example: if used for gaming applications, you can use credit to buy special weapons FB which is only offered if you use FB Credit. If Credit FB runs out, you can refill again, for example through PayPal.

Facebook have some applications and games and purchasing facility. whenever you use any application or game or others concerned thing you get credit