The custom of veiling crosses and images during the last part of Lent has changed over the centuries.
According to Adolf Adam in The Liturgical Year, in the eleventh century a cloth, called the "hunger cloth," was suspended in front of the altar beginning with the fifth Sunday of Lent. At the beginning of Lent public sinners were excluded from Church. The "hunger cloth" may have been an acknowledgment that we all are sinners and are partaking in a "fast of the eyes."
By the end of the thirteenth century statues, crosses, and pictures were veiled. French Bishop William Durandus explained it by saying Jesus veiled his divinity during his passion and that the Gospel of the fifth Sunday of Lent ended by telling us "Jesus hid and went out of the temple area" (John 8:59).
Later writers would tell us the veiling was to remind us of Jesus' humiliation and to imprint the image of the crucified Christ more deeply on our hearts.