The word altar appears twenty-four times in the New Testament. In Hebrews 13:10, St. Paul speaks of Christians having an altar where those who did not believe in Jesus could not partake. The Eucharist which happens on the altar, refers to the eternal, once-for all sacrifice of Jesus Christ, which is made "present again". It is a re-presentation of the one sacrifice made by Jesus Christ.
There is fixed altar in every church, since it more clearly and permanently signifies Christ as the living stone (1 Peter 2:4; Ephesians 2:20). The reason an altar is called 'fixed' is because it attached to the floor so as to be irremovable.
Roman Catholicism requires that there be only one altar in a newly built church, and that it be made of stone, ideally made of natural stone, such as granite or marble, as the altar symbolises Christ who is regarded as being the cornerstone of the Church. In practice, however, solid and well-crafted wood is often used, due to the expense of stone. It is still customary to place relics of saints, specifically those of martyrs, under the altar (as Mass in early Christianity was customarily celebrated above tombs of martyrs ).
Because the altar represents Christ, only what is required for the celebration of the Mass may be placed on the mensa (the flat, horizontal surface of the altar). Candles, which are required at every Catholic liturgical service, are placed either on or around the altar in a way suited to the design of the altar and the sanctuary. Catholics also place a cross, or crucifix (a cross with the figure of Christ), on the altar or near the altar, where it is clearly visible to the congregation.
General Instruction from the Roman Missal
Happy Sacred Sunday