This isn’t a separate account, but rather a function of the loan itself. A redraw facility allows you to withdraw any extra repayments you’ve made on your mortgage. Only the amount above your required minimum repayment sits in a redraw “pool” and can be accessed if needed.
For example, if you have made $1000 in extra repayments to your loan, that $1000 reduces your interest bearing balance. When you need some extra cash, you can redraw that $1000.
Often loans will have a minimum redraw amount, and a maximum number of free redraws per year. It may also take a few business days to process and sometimes incur a per-transaction fee.