Altar cloths by Paul Turner
The altar used for Mass is covered with a cloth.  At home and in restaurants, we often cover our tables, especially for a banquet or meal of some importance.  Similarly, the altar is covered because of "the banquet in which the Body and Blood of the Lord are offered"

(General Instruction of the Roman Missal 304).
But the altar is covered for another reason: "out of reverence for the celebration of the memorial of the Lord." We dignify the altar where we will solemnly remember the mystery of Jesus' death and resurrection.  When a new altar is dedicated for a church, it is sprinkled with holy water, smeared with chrism, incensed, and then covered with a cloth. Many of these symbols also appear in the baptismal liturgy, suggesting that the altar is covered in a garment just as the faithful are, as a sign of eternal life.  There are no rules governing the material to be used. The size, shape, and decoration of the cloth are to be "in keeping with the altar's design."

At least one white cloth must be used. In the United States, cloths of other colors may also adorn the altar, but the topmost must be white. It may cover the top only; it need not dangle over the other cloths.  After the Mass of the Lord's Supper on Holy Thursday, the altar cloths are completely removed. 

Good Friday liturgy begins with a bare altar. One or more cloths are placed on the altar just before the communion rite, but they are removed again immediately after the Good Friday service, and the altar stands bare until the beginning of the Easter Vigil.