The name Tenebrae (the Latin name for "darkness" or "shadow") has for centuries applied to the ancient monastic night and early morning services (Office of Readings and Morning Prayer) of the last three days of Holy Week.
Apart from the chant of the Lamentations, in which each verse is introduced by a successive letter of the Hebrew alphabet, the most distinguishing feature of the service is the gradual extinguishing of candles and other lights in the church until only a single candle, considered a symbol of Our Lord, remains. Toward the end of the service, this candle is hidden, typifying the apparent victory of the forces of evil. At the very end, the hidden candle is restored to its place, and by its light, all depart in silence.
(Developed from an explanation in "The Book of Occasional Services - 1991)