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 one purposes: to repay debt that is owed on a RuneScape account. They were also used to buy WoLCash in War of Legends before the game closed. On 22 August 2017 they were replaced by an unpaid balance with the release of the new payment flow.
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, Bonds, Treasure Hunter keys 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.