What does Hebrews 13:9 mean?

This command follows the important point made in the prior verse. The book of Hebrews explains in great detail that God's will does not change. The new covenant is not a "plan B;" it was something God intended from the very beginning. Likewise, Jesus Christ, and His gospel, are the same throughout all of history. The heroes of the faith mentioned in chapter 11, as well as the spiritual leaders of more recent past, were teaching the same story, at least from their perspective, and the same good news.

Here, the implication is made explicitly clear: changing doctrine is false doctrine. The gospel was delivered "once for all" (Jude 1:3), and those who attempt to change it are subject to a curse (Galatians 1:8–9). As imperfect people, we need to admit the possibility that we might have misinterpreted God's Word, or His will. So a "new" teaching that improves our understanding of what God truly meant is not a bad thing (Acts 17:11; 1 Thessalonians 5:21). On the other hand, any teaching different from what was originally given is false, by definition.

This verse also contains a strangely specific reference to foods and those who gain no special benefit from them. It's possible the writer has a particular group in mind. This sect might have focused on eating or avoiding specific foods. Colossians 2:16 brings up a similar point. That being said, the following verses make specific reference to the tabernacle and the Levitical system. The faction hinted at here might have claimed supernatural value of the foods given as offerings in the temple. Or, they might have simply been devoted to the system of animal sacrifices.

In any case, the point made here flows from the constancy of the gospel back into the main theme of the book of Hebrews. Namely, that the covenant offered to us by Jesus is far superior to that provided in human temples.