A standing charge is a fixed amount you pay every day you are connected to a gas or electricity network. You pay this regardless if you are on a prepayment meter or a monthly bill.
Standing charges are used to cover costs like meter reading, maintenance and the cost of keeping you connected to the network, as well as in the case of gas, the gas emergency service.
As prepayment meters apply the standing charge on a daily basis to you need to regularly top up your credit, even if you aren’t using much energy e.g. using less gas during the summer months due to not using central heating. If you run out of credit the standing charge will continue to be applied and will build up a debt which you must repay.