The explanation is in the terminology itself.

Secondary means that if you have other collectible auto insurance you must file a claim with that carrier, and the CDW bundled with the credit card pays only what was not paid by the primary insurer, such as the deductible.

Primary means that you may file a claim directly with the credit card policy without regard to any other insurance that you may carry.

Typically, USA-issued automobile policies do not provide coverage outside of the USA. This means that most USA residents do not have an collectible insurance for international rentals, and the distinction between primary and secondary is irrelevant. It is also irrelevant if you do not have an automobile policy because, for example, you do not own or lease a car.

HOWEVER, bear in mind that CDW only pays for damage to the rental car itself. It does not pay for damage to another car, other property, personal injury, legal defense etc.

Extensive discussion here:

http://www.flyertalk.com/forum/credi...ance-wiki.html