When you check in, the CM will ask if you want charging privileges on your room key. To do so, you'll need a credit or debit card with the proper amount available, or that amount of cash.
Someone above posted the limits per type of resort. Keep in mind you DON'T need to allow that much of a limit. I wanted to know how my spending was going, so even though I stayed at a moderate and could have a $1,000 limit I opted to make my limit $500. Your card does not get charged until you reach that limit. Once you reach it, you can't use your room key to charge until you go to your resort's front desk and, well, "recharge" your room key.
For two adults and two kids, we lasted from Sunday morning until about mid-day Friday on that $500 (we'd prepaid for Hoop Dee Doo, I used 5 counter-service vouchers that some kind board poster mailed to me, and our kids had about $200 in Disney Dollars to spend on souvenirs). We ate counter service (by choice, not because of money matters) except for one character dinner at Crystal Palace. After my room charge was declined on Friday afternoon at a souvenir kiosk in Tommorowland, I paid cash or charged the rest of the time since we were leaving the next day.
It was soooo convenient to not have to carry much cash, and also to have only one $500 debit on my bank statement rather than 30 or 40 individual charge transactions to try to keep track of. By the way, you can go to Guest Services in your resort at any time to get an up-to-date print-out of your room charges, if you like to keep track of those things. In addition, EVERY place you can use your room charge will give you a receipt for your transaction.