Tenebrae, Latin for "darkness," is held during the days preceding Easter and is marked by the gradual extinguishing of candles. The candles symbolize the light of Jesus, slowly but surely, fighting to overcome the darkness. The closer to Good Friday, the less light there is and on Good Friday itself even the centre candle is extinguished, in addition to all of the exterior candles.
This central candle, also known as the Christ Candle, represents Jesus who we as Christians believe to be our Saviour: the pure light. Each of the outer candles has a Lament attached to it. During Lent, First Met presented a series of Laments which can be read and/or downloaded below.
(Image from stfrancisxavierchurch.blogspot.ca)