It means "Counter Discount Program".

Here is their description:
"A counter discount program, or "CDP", is a contract or program that has been established in cooperation with a company or organization under which individuals with specified characteristics are entitled to rent on rental terms that differ from those generally offered to the public. Examples of CDP’s include corporate rate plans and credit card and travel club discount programs. CDP’s are identified by using unique numbers ("CDP Numbers"), and require those who wish to rent under the terms of a CDP to furnish Hertz with the appropriate CDP Number. A CDP Number consists of numerical values and is between 1 and 10 digits long. This number can also be used in conjunction with a PC (Promotion Code), RQ (Rate Code) or CV (Convention Number) if the special offer or promotion requires additional information is included in the reservation."