The veil of the temple … torn in two might have been one of two curtains. It may have been the curtain that separated the sanctuary from the courtyard. This curtain was a large and beautiful tapestry about 23 meters tall (80 feet). Or it may have been the inner curtain that separated the Most Holy Place (the most sacred room) from the rest of the sanctuary.
If it was the outer curtain, people could have seen it tear (compare the "darkness" in 15:33). This would show that the prediction of Jesus about the destruction of the sanctuary (14:58; 15:29) was already beginning to happen in a spiritual way (see Rom 9–11). The full physical destruction would come later, in AD 70.
If it was the inner curtain, the tearing likely had a different meaning. Just as the heavens opened at the baptism of Jesus to show his direct access to God (see study note on Mark 1:10), his death now made that access possible for his followers (see Hebrews 6:19–20; 9:3–14; 10:19–20).