Contract Portfolio is a concept discussed in proximity to the customer and service portfolio in ITIL v3. Within the Service Strategy text the contract portfolio is positioned as a concept that is derived from the "interaction" of the service and customer portfolio and is only subject to change when either of the parent portfolios change.
The contract portfolio as the name implies is a holding space for contracts and agreements related to the provision of services.
Literally the contracts held in the Contract Portfolio lay out the terms and conditions associated with the provision of services. They specify the utility and warranty requirements of the service.
Once a contract is defined it is then necessary to utilize both customer and service assets to aid in the development of the service model. The service model is the "plan" that is required in order to draw together various resources (service assets) and activities to provide the service itself.