What does Hebrews 8:9 mean?

This passage comes from Jeremiah 31:31–34, where that Old Testament prophet records God's promise to create a "new covenant" with the people of Israel. The language of this passage is extremely clear: God's intent is to provide something new, something different from the existing Levitical priesthood. This quotation is used to support the main assertion of the author of Hebrews: that God has always intended for salvation to come through Christ, and not through the Old Testament Law.

This passage repeats the idea that this promised covenant is something new. This is not the same arrangement which God made with the "fathers" of Israel, meaning the people who left Egypt during the Exodus. As earlier verses pointed out, if God is planning to make something new in the future, it necessarily means that God does not consider the "old" system to be perfect. And, in fact, it is not. This quotation from Jeremiah explains how the new covenant, fulfilled in Christ, is superior.

One of the flaws noted in the old covenant—the primary flaw, in fact—is the lack of obedience from God's people. Even though Scripture indicates that the new covenant is superior, all of the drawbacks of the old covenant are rooted in human flaws (Hebrews 8:8; Romans 3:20). Priesthood is inherited (Hebrews 7:20), priests die (Hebrews 7:23), priests sin (Hebrews 7:27). Here, the fact that the people disobey and defy the covenant is mentioned. This, for Israel, came with the very consequences promised them by God (Deuteronomy 30:15–19). The new covenant is superior, primarily, because it is grounded in the work of Jesus Christ, rather than in fallible human efforts (Galatians 2:16).