These images were taken in Italy and France. This example is a simple green wool felt with a golden silk galloon as a border, in order to finish the cloth with a proper trim. All altars in the same church should have the same matching cover. There is also a certain way of folding it so as to keep the bottom free from any possible dust, and to make sure the top never touches the altar clothes.
The cloth has symbolism and a practical use. It indicates the sanctuary is at rest. In addition, it not only protects the altar from dust. In summer months it can also protect it from bat droppings or even mouse droppings. In the Byzantine tradition, there is the "aer," a large rectangular veil used to cover the chalice and paten, symbolizing the Holy Spirit or the burial shroud of Christ. It essentially serves the same purpose when placed over the Gospel and antimension.
Sometimes a similar cover can be seen in sacristies on the top of vesting tables where vestments are laid out. These coverings are generally not purchased in a shop. They are custom made, often by lay parishioners or nuns. Today suitable fabric of any number of colors and golden French galloons can be purchased online for anyone who may wish to make one at home and donate it to their parish.
The altar symbolizes Christ, the rock. The impulse to adorn the altar is from an impulse of love for the Eucharist. Reverence leads the Christian to a deeper sense of the sacred. All of the extras, including the dust cover for altars, point to the real presence and the worship of the One True God, made Incarnate and dwelling upon the sacred altar of sacrifice.
