"Therefore, since we have a great high priest who has gone through the heavens, Jesus the Son of God, let us hold firmly to the faith we profess." NIV translation
The job of the high priest was to make sacrifices for the sins of the people because, as we saw, all sin will be judged. Jesus is the ultimate high priest in that He is the Son of God and gave Himself as the perfect sacrifice. Paul urges us to hold onto the fact that Jesus' sacrifice was sufficient. Mankind continues to try to add works as if His sacrifice was not enough to pay for all of our sins but we must resist this temptation and the legalism that accompanies it.
"For we do not have a high priest who is unable to sympathize with our weaknesses, but we have one who has been tempted in every way, just as we are - yet was without sin." NIV translation
Jesus came to earth as a baby and lived through the same temptations that we face every day. He did so without sinning so that He could be the perfect sacrifice. Because of the fact that He was tempted, He can understand the struggles that we face.
"Let us then approach the throne of grace with confidence, so that we may receive mercy and find grace to help us in our time of need." NIV translation
We can go to God ("the throne of grace") with confidence because we know that Jesus paid for all of our sin and He understands us. He does not count our sins against us (mercy) but instead gives us forgiveness & salvation (grace). When He said that it was finished, He meant that all of our sin was paid for and we can be sure of that and live in that victory. There are too many people (even Christians) walking around in fear of losing their salvation because they stumble and think that they have finally committed one too many sins. Our confidence is in the fact that Jesus paid for our sins of today & tomorrow as well as those of our past.