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They were told not to damage the grass on the earth or any green plant or tree
Ordinary locusts were a terrible threat to people because when they swarm, they can eat up all the grass and all the leaves on plants and trees. These locusts were told not to do this.
but only the people
The phrase "to damage" or "to harm" is understood. Alternate translation: "but only to harm the people" (See: rc://en/ta/man/jit/figs-ellipsis)
the seal of God
The word "seal" refers a mark that would be left by a tool being pressed into a a wax seal. In this case it refers to mark that God would place upon the foreheads of his people. The people who do not have this mark do not belong to God. See how you translated "seal" in Revelation 7:3. Alternate translation: "the mark of God" or "stamp of God" (See: rc://en/ta/man/jit/figs-metonymy)
foreheads
The forehead is the top of the face, above the eyes.