The altar is the table on which the Eucharist is celebrated and the structural centre of a church, and Christian tradition has held it to carry two meanings at once.
Table and altar
The Eucharist is both a meal and a sacrifice in Christian understanding, and the object at its centre is spoken of as both a table, around which the supper of the Lord is shared, and an altar, on which a sacrifice is offered, the emphasis differing between traditions that stress the one sense or the other. The Reformation argument over whether the rite is chiefly a supper or a sacrifice was fought partly over the word and the form of the object, some reformed churches deliberately using a plain communion table set in the body of the church rather than a fixed stone altar against the east wall.
Relic and consecration
The Catholic altar has traditionally been consecrated with chrism and has contained the relic of a saint, a practice that reaches back to the celebration of the Eucharist over the tombs of the martyrs in the early church, so the altar carries the memory of that origin and joins the sacrifice on it to the witness of the saints beneath it. A fixed altar of stone is consecrated and set apart permanently, becoming the fixed centre from which the whole building is oriented. The altar thus gathers into one object the meal, the sacrifice and the memory of the martyrs, which is why it stands at the heart of the church.