Jesus is in a perfect position to offer such a sacrifice for sin. Having taken human form, Jesus has sympathy for the lives that people live and, due to this, wants to approach God to help them. However, Jesus is also more equal to God than human beings and can approach God more freely and directly. The sacrifice that Jesus offers as high priest is not an animal, though; it is his own death.
[links to Bible Gateway open in a new window]The second reading offers a warning about those who accept this teaching and then later turn away from it. If one has been forgiven, but then stops being a Christian, one cannot later be saved. The reason for this is directly related to the understanding of Jesus being the perfect sacrifice that completely atones for human sin. The sacrifice that atones for sins does so once; if there are other sins after that time, then another sacrifice will be necessary (which was why the high priest offered such a sacrifice on the people's behalf once each year). However, Jesus cannot be sacrificed again.
This is meant to encourage people to be diligent and persevering in their faith, despite the challenges and obstacles. How awful it would be to find salvation and hope through Jesus, only to lose those things foolishly.
[links to Bible Gateway open in a new window]Reading the Bible in Its Entirety
If you are planning to read the Bible in its entirety, you should read Hebrews 4-6 today.