The "Available Now" section of your Hilton Honors Debit Card app shows how much you are able to spend using your card, taking into account your Spending Limits, any pending payments (i.e., ones that have not been taken from your linked account yet), pre-authorisations, refunds, and any overdraft facility that your linked account has.
Your "Available Now" balance may not always reflect the balance of your linked bank account and is only designed to show how much you're able to spend using your card.