A type of trade contract, meaning the supplier will pay all the costs involved in delivering the item to the buyer, including freight, insurance and any Customs duties. Note that DDP is a legal term and the exact meaning differs from country to country.
When importing, during your negotiating, your goal should be to get the supplier to take on as much responsibility for the shipment as possible. For example, it is beneficial for you if the supplier is responsible for the shipment from the point it leaves their warehouse, to the point it arrives at your doorstep. However, for smaller shipments, many suppliers rid themselves of all responsibility when it leaves their warehouse. This is commonplace.
For large items ordered direct from suppliers located internationally, it's best to consult a Customs Broker to handle customs duties and taxes.