Spanish for "altar piece", retablos are a tradition in folk art where religious scenes or images are painted on inexpensive material (for example, wood, or thin tin and iron sheets), or crafted into small ornamental wooden boxes placed about the home or church. The practice is of European origin, and was transplanted to the western hemisphere by the Spanish conquistadores. These icons are meant as a form of religious devotion, or, as a remnant of pre-conquest traditions, as good luck symbols.

Several themes are common in retablo art, with the crucifixion of Christ, The Virgin Mary and Child, and Virgin of Guadalupe being particularly common, along with a host of saints and apparitions. They may also express gratitude for being cured of illness or worry by religious devotion.

Retablo boxes are found throughout Central and South America and are often very finely crafted, with doors opening onto a richly detailed scene. The paintings on tin are indigenous to Mexico, where the raw tin sheets were common and inexpensive. The practice of hand-crafting retablo paintings faded at the end of the nineteenth century when the inexpensive mass production of pictures and engravings became possible.

A collection of Mexican retablo paintings can be found at http://www.nmsu.edu/~artdept/retablo-web/, from which some information for this writeup was taken. The New Mexico State University art museum has an extensive collection of them as well if you happen to be in the Las Cruces area.

Supposedly, retablo also refers to a fairly common type of art (at least at the time of Frida Kahlo, who was known for collecting and painting retablos in addition to her other work). It is a depiction of a crisis in someone's (usually the artist's) life. The drawing is generally fairly simple, often of a realistic or somewhat cubist style, telling the story of the moment of the crisis.

The piece is often fairly small, and as Grzcyrgba said, painted on wood or tin, whatever happens to be available and preferable to the artist. Beneath the painting itself is a passge, generally handwritten by the author in his or her native language, describing the accident and giving thanks to their deity of choice (also as Grzcyrgba said, there is usually a religious overtone to a retablo and thus thanks is generally given to God) that this specific indicdent was not as bad as it could have been, and that they survived the ordeal and lived to tell about it.

Frida Kahlo was injured in a public transit accident when she was a teenager. She painted several retablos referring to this incident, and she also enjoyed collecting the retablos that others had made.