This happens when the payment processor we use, Stripe, verifies the credit card details you provided on Grabr.
First, Stripe will attempt a $0 authorization to verify your credit card details (except AMEX, which does not support the $0 authorization). If the $0 verification check does not work, Stripe will then attempt a $1 temporary authorization.
Please note that this is only a temporary authorization charge. Your card issuer will remove it from your credit statement, usually within a few days.