"The devourer" simply means "the one who devours" (or destroys). It refers to anyone or anything known for devouring. In this passage, it refers to whomever or whatever "devoured" or "ate up" or "destroyed" the "fruits" and "vines".
In particular, it does not refer to anything or anyone in particular. It leaves who/what devours as undefined, placing the emphasis on the simple fact that something was or is being devoured.
This has likely reference to Joel 1:4 and 2:25. But, it also has bearing on the final verse of this book, which is the final verse of the Old Testament, Mal 4:6, indicating that God would be the one who decrees the curse that "strikes the land". So, this "devourer" (3:11) was probably sent by God and therefore God can also rebuke/stop this "devourer", whomever or whatever God may have decided that the "devourer" should be.
It also has reference to Job, who suffered from a "devourer" like this first in the Bible.
The "roaring lion" who "seeks whom he may devour", Satan, in 1 Peter 5:8 is a "devourer", but not all devourers are necessarily Satan. There are many devourers that are not Satan. Peter came about 400 years after Malachi, so Peter is putting Satan in the "devourer" category Malachi already explained. The relationship is like squares and rectangles. Later passages in the Bible refer to the former, not vice versa. Whether this "devourer" is Satan or the wind is unstated on purpose because that is not the point.
In fact, by leaving the "devourer" unspecified, it's almost Malachi's point that "who the devourer is" is not the point.
In applying this passage to our lives, sometimes our "devourer" is Satan, sometimes it is not, but the important part is that God will rebuke any devourer based on what happens between us and God. The devourer is merely a tool—God's tool—and that's how Malachi treats it.
To understand Malachi's point here, we must understand "rebuke", which basically means God will "stop" the devourer, thereby stopping the devouring and, by implication, restore what the devour took (Joel 2:25). Just as Job's accuser is never named and is simply forgotten at the end of that story, the emphasis is not on this "devourer", but on God's power to stop the damage, to restore the damage, and our choice to ask Him to.