What does Malachi 3:9 mean?

According to the covenant between God and Israel, the people were to bring 10 percent of everything they possessed to the temple. This not only supported the Levites, who maintained the temple, it was also used to care for widows, orphans, and others (Leviticus 27:30). Verse 8 indicated that Israel has been "robbing" God by keeping these tithes and offerings. Since Israel was violating the covenant, they could expect the promised curses (Deuteronomy 28:15–68).

Verses 10 and 11 describe God protecting Israel from economic hardship, if they will honor the covenant. So, the curse being mentioned here is likely agricultural. Given the complaints of the people in this book, and their continued domination by Babylon, this was probably a time of financial and social hardship for Israel. It's common for humanity, in all generations, to defy God, then blame Him for the consequences of our own sin. In this case, that complaint is doubly hypocritical, because these very consequences were explicitly promised by God if Israel broke the covenant!

As with verse 8, and the upcoming promise of verse 10, context is key to complete understanding. Malachi's criticism, like the law of Moses, is meant for the nation of Israel. While we can draw principles for modern faith from this book, the covenant agreement being discussed here, including the requirement of tithing, is meant for the nation of Israel. This is not a regulation applied to all people at all times.