Although the answer to this could vary depending on the time period that you are asking about, the most likely answer is spices. Spices were one of the first commodities that Europeans wanted to get from Asia in large quantities. Spices from Asia were brought to Europe as long ago as Roman times. The trade continued, carried mostly through Muslim lands and then to the Mediterranean, during the Middle Ages. It was because of a desire to break the Muslim hold on the spice trade that Portugal started exploring ways to sail around Africa.
Spices were an excellent good to trade in because of the fact that small quantities, weight wise, were worth a great deal of money.