Holy Communion, also called the Holy Eucharist, is the most vital sacrament of the Church, and its nature in the Orthodox Christian faith is different to the variations of the sacrament practiced outside it. In his The Orthodox Faith series, Fr Thomas Hopko begins his explanation of the Eucharist as follows:
The Holy Eucharist is called the “sacrament of sacraments” in the Orthodox tradition. It is also called the “sacrament of the Church.” The eucharist is the center of the Church’s life. Everything in the Church leads to the eucharist, and all things flow from it. It is the completion of all of the Church’s sacraments—the source and the goal of all of the Church’s doctrines and institutions.
For more information, see the full chapter on the Orthodox Church in America website.