They are holding back the “four winds,” meaning winds from the north, south, east, and west. Earlier Scripture had already mentioned these “four winds” (see Jeremiah 49:36; Daniel 7:2). Such earlier passages make clear that these are winds of divine judgment intended to do great damage to the nations to which they are sent. So we are not surprised to read at the end of Revelation 7:2 that these four angels have power “to harm earth and sea.” For now, though, these angels must hold back these winds of judgment.
1 After this I saw four angels standing at the four corners of the earth, holding back the four winds of the earth, that no wind might blow on earth or sea or against any tree.