A standing charge is a fixed daily amount you pay for your gas and electricity.
Standing charges are used to cover the costs of supplying energy to your home. Think of it like the line rental you pay on your landline phone connection, with your calls - or your energy use - added on as extra.
You need to pay your standing charge even if you're not using much, or any, gas or electricity (like in the summer, when we have our heating and lights less often, or when you go away on holiday).
So, remember to top up enough to cover these daily charges, along with any energy you're using.