CDW is otherwise know as either "collision damage waiver" or "loss damage waiver" and its a form of insurance to protect drivers from the full costs of damages or loss of a vehicle in its simplest terms.
This form of insurance is included in almost all car rental agreements to help protect both rental car drivers and the rental providers themselves from large losses should something unfortunate happen during the rental term. The car / van renter is left with the excess on the collision damage waiver insurance (to which you can take out additional protection to cover this excess through either ourselves or the rental provider collecting the vehicle from).
If your question isn't answered here you may want to checkout these pages: