What Does Exodus 10:12 Mean?

Then the LORD said to Moses, "Stretch out your hand over the land of Egypt for the locusts, so that they may come upon the land of Egypt and eat every plant in the land, everything that the hail has left." The execution command for the Locust plague is the same staff-extension gesture used in the blood plague (Aaron's staff over the water), the gnat plague (Aaron's staff on the dust), and the hail plague (Moses' staff toward heaven).

Here Moses stretches his hand over the land of Egypt (not specifically over water or toward heaven): the complete geographic gesture matches the complete geographic scope of the locust plague that will cover the entire land.

The command specifies the locusts' diet: "every plant in the land, everything that the hail has left." The locusts are targeted specifically at the post-hail agricultural remnant. The hail destroyed the mature crops; the locusts are designed to eat the immature crops that survived. The two-plague agricultural elimination is explicitly chained: hail first, then locusts to eat what hail left. Egypt's food supply is being systematically eliminated across two plagues, each designed to destroy what the previous one spared.

The "stretch out your hand over the land of Egypt" is Moses stretching over the entire country, not over a specific resource or location. The geographic comprehensiveness of the gesture corresponds to the geographic comprehensiveness of the plague: the locusts will cover the entire land (verse 15, "the land was darkened"). The hand gesture that starts the locust plague is the widest-scope extension in the plague sequence, appropriate for the widest-scope biological devastation of the Exodus.