While I can't verify which version is more accurate, I learned a slightly different version of
retablo at
art class at
school (
yick).
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.