In order to charge back to the room, you must have a credit or debit card on file. The day you arrive, Disney will put a $100 hold (pending charge) on that card/account.
You charge to the room with your magic band throughout the day. On your way back to your room every night, you stop at the front desk and pay your balance (which you can track in the
MDE app).
A few days after checkout, the $100 pending charge will just drop off.
If you do not pay the balance every night, an additional pending charge will be added to you card/bank account in the amount of your charges, so make sure you stop in every night if you do not want that to happen.