From Wikipedia: Sealing of the holy relics In the early days of Christianity when the Church was heavily persecuted, the Christians met in underground burial places where they celebrated the Eucharist on the graves of martyred saints. After the Church was recognized this custom was continued by placing relics in the altar table during the consecration of the church. This is a reminder that the Church was built on the blood of the martyrs and their faith in the Lord.
After the bishop has entered the church, he continues into the altar. In the altar he places the diskos on the altar table. There he removes the relics and places them in a small box. The bishop then pours holy chrism over the relics, symbolizing the union between our Lord and his martyrs. With prayers and the reading of Psalm 145, the bishop then places the box with the relics in a cavity in the altar table where it is sealed in with a wax/mastic that contains fragrant spices as were used by Joseph of Arimathea and Nicodemus to anoint our Lord's body before his burial. In this, the holy altar represents Christ's tomb.
Washing and anointing the altar table
After placing the relics in the altar table (disposition), the bishop proceeds to the washing and anointing of the altar table. For this purpose the bishop is vested in a special white linen garment over his vestments called a sratchitza or savanon. The baptism of the altar table begins with the prayer of consecration by the bishop, followed by petitions by the deacon. The bishop then is given a basin of water and, with a blessing and prayer, pours the water over the table three times and washes it while Psalm 84 is read. Symbolizing baptism, the table is cleaned by washing and made holy by the grace of the Holy Spirit. After the table is dried, the bishop sprinkles rosewater on it and continues reading Psalm 51. The assisting priests then dry the table with the antimins.
Having anointed the table with chrism, once in the center and on each side, the bishop proceeds to spread the chrism over all the table while reciting a section of Psalm 133. The excess chrism is wiped off by the priests with the antimins, and icons of the four Evangelists are fastened, one at each corner, to the altar table. Vesting the altar table While Psalm 132 is read, a white linen cloth, representing the Lord's burial shroud, is laid over the altar table. The cloth, called the katasarkion, is tied on the table with a cord that represents the cord with which our Lord's hands were tied when he stood before the high priests. The katasarkion is permanently installed, to remain as long as the church stands. After washing his hands, the bishop now covers the altar table with a more ornate cover, the endyton, that symbolizes the glory of God and places the other holy articles, including the antimins, Gospel Book, the artophorion, and candle sticks, on the altar table, as the reader reads Psalm 93.