In the Sacrifice of the Mass, it is always good to remember that holy things are veiled. Before the offertory, the chalice is veiled as well as the tabernacle and all the ciboria which are in the tabernacle. These veils remind us of the ancient Holy of Holies in the temple which was also veiled (see Heb 9:1-8). We do this because in the Mass of the Catechumens (the first half ending with the Creed), the way of salvation is still obscured. But, then at the Mass of the faithful (the second half of the Mass beginning with the Offertory), we see that our salvation takes place through Christ's sacrifice and therefore the way is unveiled.