When Jesus died, the curtain in the temple, separating the Holy of Holies from the common area the priests served in, was torn in two.
While we have no context of what that means here in the Gospel of Mark, the book of Hebrews gives us a little bit more insight.
We have this hope, a sure and steadfast anchor of the soul, a hope that enters the inner shrine behind the curtain, where Jesus, a forerunner on our behalf, has entered, having become a high priest forever according to the order of Melchizedek. – Hebrews 6:19-20
The tearing of the curtain gave us direct access to God, through Christ. By His death, Jesus now serves as our High Priest. There are no barriers to having a relationship with God.
When it was noon, darkness came over the whole land[e] until three in the afternoon. At three o’clock Jesus cried out with a loud voice, “Eloi, Eloi, lema sabachthani?” which means, “My God, my God, why have you forsaken me?”[f] When some of the bystanders heard it, they said, “Listen, he is calling for Elijah.” And someone ran, filled a sponge with sour wine, put it on a stick, and gave it to him to drink, saying, “Wait, let us see whether Elijah will come to take him down.” Then Jesus gave a loud cry and breathed his last. And the curtain of the temple was torn in two, from top to bottom. – Mark 15:33-38