This excellent video from Sensus Fidelium explains it well:
From [Passion] Sunday, traditionally called 1st Sunday of the Passion, it is customary to veil images in churches. In the Gospel in traditional Form of the Roman Rite we hear:
Tulérunt ergo lápides, ut iácerent in eum: Iesus autem abscóndit se, et exívit de templo. … They therefore took up stones to cast at Him; but Jesus hid Himself, and went out from the temple.
This is a fine old tradition. It has to do with deprivation of the senses and the liturgical dying of the Church in preparation for the Lord’s tomb and resurrection. We do this to sense something of the humiliation of the Lord as he enters His Passion, something of His interior suffering.
We are also being pruned during Lent. From Septuagesima onward we lose things bit by bit in the Church’s sacred liturgy until, at the Vigil, we are even deprived of light itself. The Church is liturgically dying.
Catholicism Pure & Simple 
I was asked the other day why a young bride and her mother, friends of mine but not so close that I was invited to the wedding, were told that they could not decorate the altar with flowers as these were not allowed during Lent. I couldn’t give an answer. Can anybody help?
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I believe it is ‘The General Instruction of the Roman Missal’ (GIRM) that states flowers are not to be used to decorate the altar during Lent.
“305. During Lent it is forbidden for the altar to be decorated with flowers. Exceptions, however, are Laetare Sunday (Fourth Sunday of Lent), Solemnities, and Feasts.”
This, together with the use of the colour purple and covering the Crucifix and statues in the Church, is in order to keep the solemnity of the season of Lent.
I was at a very beautiful concert of Lenten musical sacra tonight. The stage was lit by low lights that were gradually dimmed, and candles that were distinguished one by one, as the choir moved from the Pueri Hebraeroum of Palm Sunday through to the Sepulto Domino of Holy Saturday.
It artfully created the atmosphere of a slow dying, representative of the last days of Our Lord’s earthly life.
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