Store credit means that instead of money, your card (only good at THAT store) has a certain amount of money you can spend in that store. To me, it's kind of like a gift card that can only be used at one store. I normally get store credit if I return something. Instead of actual money, they give me store credit.
In this case, your ACCOUNT (that you have with that store) has six pounds eighty left in credit. An account is normally used to track a customer. An example of this: I go shopping at a huge retail store (like Macy's in America). When I pay for my clothes, the cashier asks me if I'd like to open an account with them. It is my choice to say yes or no.
If you have any more questions, just ask! I'm sorry if it seems a little confusing. :(