Concise Bible Dictionary:
Besides the common use of this word, it occurs at the commencement of several prophecies; as “The burden of Babylon,” “the burden of Moab” (Isa. 13:1; Isa. 15:1). The learned are not agreed as to the force of massa in such places: its natural meaning would be “a judgment that lies heavy on the people;” but some take its meaning to be “an oracle or sentence pronounced against them.” The word occurs also in Jeremiah 23:33-38, where it is “the burden of the Lord.” The false prophets were not to use this expression, as if they had a message from God. If they did, it should bring judgment upon them. The same word is translated “prophecy” in Proverbs 30:1 and Proverbs 31:1.