There's a couple reasons why your prepay meter might be showing that a debt.
You need to pay this back, so when you top up, your meter will take the cost of the emergency credit you've used off the total amount of credit you've added.
A standing charge is a small daily fee that's used to cover the costs of supplying energy to your home (including things like maintaing the cables and pipes that takes electricity and gas into your home). Think of it like the line rental you pay on your landline phone connection.
Even if you haven't used any gas (like in the summer, when you probably have on your heating a lot less), you'll still have to pay a standing charge every day.
If you're not topping up enough to cover this standing charge, along with the energy that you're using, then you can build up a debt on your meter.