The antependium/altar frontal is a decorative textile that covers the front of the altar during Catholic Mass.
The example in the featured photo (above) is embroidered with gold, silver, silver-gilt and silk threads on applied pieces of silk. Scenes of the Baptism of Christ by St John and the Virgin and Child with the infant St John are depicted (Victoria and Albert Museum).
The altar represents Christ and the antependium is His clothing, so the materials used to create them are usually chosen by color according to the liturgical calendar and make the altar central to celebrations of Mass. The green antependium and chasuble in the photo below reflects ordinary time in the liturgical calendar; this is when there is regular Mass, but it is not a High Holy Day.
Antependium are often coordinated with the chasuble of the Mass celebrant, as above, so there could be many different patterns, colors and designs that would coordinate with the specific celebration.
Vestments, including antependium and chasuble, would be carried by the clergy traveling with Coronado’s Expedition. The antependium would allow them to turn a makeshift surface, such that they could find while traveling, into a proper altar representative of Christ as the center of the celebration of Mass.
Altar Frontal (Spain), 16th century; silk, metallic thread, garnets; H x W x D: 97.8 x 265.7 cm (38 1/2 in. x 8 ft. 8 5/8 in.); 1937-31-1. Retrieved March 20, 2019 from Cooper Hewitt, Smithsonian Design Museum, Textiles Department, http://cprhw.tt/o/2C29D/.
Altar Frontal (Spain), 1530; Embroidered silk velvet with gold, silver, silver-gilt and silk threads; T.141-1969. Retrieved April 23, 2019 from Victoria and Albert Museum, Textiles and Fashion Collection, https://collections.vam.ac.uk/item/O93215/altar-frontal-unknown/.
Catholic calendar 1550. (nd). Universalis Publishing. Retrieved March 28, 2019 from http://universalis.com/calendar.htm.
Dipippo, Gregory. (4 August 2016), The History, Development and Symbolism of the Antependium, Altar Frontal or “Pallium Altaris”. New Liturgical Movement. Retrieved April 29, 2019 from http://www.newliturgicalmovement.org/2016/08/the-history-development-and-symbolism.html#.XNrUrI5KjIU
Schulte, A.J. (1907). Altar Frontal. In The Catholic Encyclopedia. New York: Robert Appleton Company. Retrieved March 28, 2019 from New Advent: http://www.newadvent.org/cathen/01353b.htm.