For example, we could conceivably all grow our own food and provide our own medical care. But because farming and medicine require special knowledge and skills, a far more efficient arrangement is for the farmer to specialize in growing food and for the doctor to specialize in treating patients.
Through the specialization made possible by trade, the farmer can benefit from the doctor's medical knowledge and the doctor can enjoy lunch. The opportunity to trade allows everyone to play to his or her own strengths while benefiting from the productive skills of the whole community.