The transportation industry, like the construction industry, has many situations where a bond is required in order to obtain work or a permit to operate.
Bonds are basically the insurance company guaranteeing that you will perform or pay as required under your obligations. In the event an obligation is not honored, a claim by the other party can be made against your bond. The insurance company will then seek reimbursement from you if the claim was valid. Therefore, bonds are, in essence, a form of credit.