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.