What does Malachi 2:4 mean?

God never does anything without a reason, and rarely without some specific message in mind. In the prior verses, Malachi has delivered a dire warning from God to the priests of Israel. If they do not correct their insulting, improper worship practices, God will humiliate them. He will do this by debasing them in the eyes of others, and by removing that which is impure (Malachi 2:3). The symbolism given in verse 3 was the "dung" or "offal" from the temple sacrifices—the filthy and unusable parts which were discarded prior to offering a valid sacrifice.

As explained here, this is not being done merely for revenge. God's intent is to purify the Levitical priesthood, in order to fulfill the promise He made to the tribe of Levi (Numbers 18). As a prophecy of something yet to come, this threat is also meant to prove God's involvement. Seeing the priesthood humbled and purified would demonstrate to Israel that He was honoring His agreement with the tribe of Levi.