What is an instant transfer to a debit card

It is a transfer from your Apple Cash to your debit card. There is a small fee -- see below from Transfer money in Apple Cash to your bank account or Visa debit card - Apple Support

A 1 percent fee (with a minimum fee of $0.25 and a maximum fee of $10) is deducted from the amount of each Instant Transfer.

You can transfer from Apple Cash to a bank account in 1 to 3 days for no fee.