The High Priest in the tabernacle (and later the temple) was set-apart by God. He was from the lineage of Aaron from the tribe of Levi. The tribe of Levi produced all the priests who were to minister day and night before God.
There was only one High Priest in the nation at any one time. He was chosen and consecrated in an elaborate ceremony. He oversaw all the priests and had many duties. One of the ways he shadowed Jesus was to act on behalf of men in relation to God. He represented the people by offering gifts and sacrifices for sins. But the most important role was a key one on the ‘Day of atonement’ (also called Yom Kippur).
The ‘Day of Atonement’ was the day in which sacrifices were made to cover the sins of the people and restore their relationship with God.
The High Priest would sacrifice a bull to cover his own sins and then, with great trembling, would pass through the curtain into the holy of holies, into the very presence of God and sprinkle the sacrificial blood. Would his gift be accepted? Would he make it out alive? This would be repeated every year by the High Priest of that year.
This whole procedure was clearly looking forward to the Great High Priest who would deal with sin once and for all time. As I mentioned earlier, this is set out in the book of Hebrews;
And every priest stands daily at his service, offering repeatedly the same sacrifices, which can never take away sins. But when Christ had offered for all time a single sacrifice for sins, he sat down at the right hand of God, waiting from that time until his enemies should be made a footstool for his feet. For by a single offering he has perfected for all time those who are being sanctified. (Hebrews 10:11-14)
Jesus made the perfect sacrifice, one that never needs to be repeated. We now have access to the very presence of God. We can enter more than once a year. We can come boldly, without fear and trembling, any time we like, because of His ultimate sacrifice.