There are two common meanings of the word reredos. In general architecture, the word can mean the back of an open hearth of a fireplace or a screen placed behind a table.
In more common religious usage, a reredos (also spelled raredos) is a screen or decoration behind the altar in a church, usually depicting religious iconography or images. It can be made of stone, wood, metal, ivory, or a combination of materials. The images may be painted, carved, gilded, composed of mosaics, and/or embedded with niches for statues. Sometimes a tapestry is used, or other fabric such as silk or velvet.
The term is derived from the a Middle English term which is derived from an Anglo-Norman 14th century term areredos, from arere behind +dos back, from Latin dorsum. In French and sometimes in English, this is called a retable (in Spain a retablo etc).
The retable may have become part of the reredos when an altar was moved away from the wall. For altars that are still against the wall, the retable often sits on top of the altar, at the back, particularly when there is no reredos (a dossal curtain or something similar is used instead). The retable is also where the altar cross, flowers and "office light" type candlesticks sit.
Although the term dates back to the 14th and 15th centuries, it was nearly obsolete until revived in the 19th century.