Recall how Moses noticed a burning bush on top of Mount Horeb (also called Mt. Sinai). The flames of the fire did not consume the bush. This led Moses to go up the mountain, where he encountered and spoke to God. The burning bush symbolized the presence of God; that Moses was on holy ground. In Catholic Churches, the candle by the Tabernacle is lit at all times to symbolize the burning bush and to remind us that Jesus/God is truly present in the Tabernacle and that we are on holy ground. It is only on Good Friday, when the Tabernacle is emptied, that we do not have the red candle burning by the Tabernacle.