Prior to the Divine Liturgy, the bread and wine used for the service are prepared in what is known as the Proskomedia (Proskomide) or the Liturgy of Preparation. Because this service takes place behind the iconostas, its rich symbolism is sometimes not appreciated by the people who attend. There are a few ways to learn more about this service. Much is available online in articles at various sources but there are also some excellent videos on the Proskomedia service that have been produced worth watching. Thus, it is possible to see the service as it is traditionally performed at the altar of prothesis behind the iconostas as well as watch explanations of the service.
For example, here is a video recording of the service from St George Greek Orthodox Cathedral in Greenville, South Carolina:
Another way to learn more about the proskomedia is from catechetical presentations by clergy. Here Fr. John Peck of St George Greek Orthodox Church in Prescott, Arizona presents an instructional proskomedia to members of his parish explaining some of its spiritual significance:
For further reading and viewing:
Of Prosphoras and Pre-Cut Pieces (contrasts the traditional Orthodox proskomedia service with the latinized shortcut service done in many Eastern Catholic parishes)