Hebrews 10:8-10 meaning

After quoting Psalm 40:6-8, the Pauline Author explains his reasoning for the reference: After saying above, "Sacrifices and offerings and whole burnt offerings and sacrifices for sin You have not desired, nor have You taken pleasure in them" (which are offered according to the Law), then He said, "Behold, I have come to do Your will." He takes away the first in order to establish the second (vv 8-9). 

His argument is that if sacrifices and offerings (made under the law) are not what God desires, and if Jesus came to do God's will, then the first covenant is displaced, and a new one, the second, has been established. That means we are no longer subject to the first covenant, including the ceremonial rules. 

Jesus offered His body by dying as the final, perfect sacrifice, which was what God really wanted. Through this final offering, we are sanctified (made holy/justified in the sight of God). As written here, we have been sanctified points to the completed action in the past (Christ's sacrifice) of which the effects go on forever. By this will we have been sanctified through the offering of the body of Jesus Christ once for all (v 10). 

There is another type of sanctification, where we become Christ-like if we live faithfully, but that is a progressive sanctification. That is spiritual maturity. What is being talked about here is a permanent, completed sanctification. This is the gift of justification salvation, where we are declared righteous before God's throne forever, because of Christ's sacrifice. Animal offerings don't cleanse anyone of sin, but the body of Jesus Christ, offered once for all, cleanses everything. In one sacrifice, sin is removed as a barrier between us and God. And this amazing gift is received by any person who believes once, for all time.

The fact that His sacrifice was once for all shows that it is a better sacrifice than the animal sacrifices. One sacrifice, one time, that sanctifies all who believe. There is no need for repetition. Christ does not have to die again and again, like the burnt offerings of the Old Testament. 

Through His death and resurrection, the Old Covenant is done, and the New Covenant is established forever. The reason the Pauline Author is belaboring an emphasis on animal sacrifice is to make a point later in this chapter that relying on animal sacrifices to cover willful sin is the height of folly and will be accompanied by great loss.