Some Grubhub orders require that the driver pays for the order at the restaurant. The Grubhub Driver Card is sent to Drivers by Grubhub so that these orders can be paid for by Grubhub.
These cards arrive in the mail and are activated by the driver directly in the Grubhub for Drivers app.
You should never pay for any Grubhub order that you have accepted for delivery with any form of payment other than the Grubhub Driver Card. Drivers will not be reimbursed by Grubhub for any other form of payment.