JCoins are a form of virtual currency made by Jagex and first introduced with War of Legends and are obtained by buying them with real money. They only have two purposes: to buy WoLCash in War of Legends and to repay debt that is owed on a RuneScape account.
When a player disputes a payment and the money is returned to the bill payer, negative JCoins are added to a RuneScape account because the cost of membership, Squeal of Fortune spins or RuneCoins has been returned to the bill payer, turning the used membership into a debt.
Before the RuneScape account can be played again, any membership used will need to be paid off. This can be done by either purchasing JCoins or reversing the disputed payment.