This verse is about God promise’s to protect and provide for us if we tithe has he commanded. God would not only give a fruitful season, so that the crops sprang up well, but would guard them from everything that could injure them before they were gathered in. It is in this context that God makes the promise to rebuke the devourer (the locust of the field) on behalf of the people of Israel. If they would repent and return to Him, He would pour out blessings upon them. If they would bring into the storehouse the grain offerings that are rightly His, He would open the heavens and pour out blessings upon them for their obedience. Their crops would produce so abundantly that there would scarcely be room enough to store them (Malachi 3:10).
Some have tried to compare God’s rebuking of the devourer with Christians’ rebuking of the devil. But this is a misinterpretation and takes the verse out of context. The “devourer” is not Satan; it is literally the locust who would eat up the fields. The church is not Israel; we are not under the Old Testament Law; We are under Grace and Jesus released us from the Law by his sacrifice. Of course, we do have a spiritual enemy. Satan prowls around seeking whom he may devour (1 Peter 5:8), but even if it were referring to Satan, which it is not, the verse is clear that it is God who does the rebuking ( Zechariah 3:2 and Jude 1:9). The Bible does not give Christians the authority to rebuke the devil or to speak to him at all. We are told to resist him and he will flee from us (James 4:7).
This verse along with verse 10 simply tells us we are to tithe and in doing so God will protect us and provide for us as a father should.