What is a perpetual light in a Roman Catholic church?

"Perpetual Light" is probably another name for the red sanctuary light that is over the Tabernacle. This light (usually a large candle, and always red) is kept lit 24/7 and the only time it is put out is at the end of Maundy Thursday Mass where it is moved, along with the Blessed Sacrament, to the Altar of Repose until Easter Vigil when it is brought back to the Sanctuary.

The purpose of this light is to show that Christ (our Light, the Light of the World) is present in the Blessed Sacrament, and therefore in the Tabernacle.