From the article: "Even American Express Co., which historically has focused on customers who pay their bills off every month, is now concentrating on lending

From the article: "Even American Express Co., which historically has focused on customers who pay their bills off every month, is now concentrating on lending money to consumers who keep a balance. Outstanding balances reached nearly $952 billion in the first quarter, ..." This seems to imply that, by "keep a balance," they mean not paying your bill off every month.