What does Exodus 10:4 mean?

Moses and Aaron predict an eighth plague to Pharaoh (Exodus 7:21; 8:6, 17, 24; 9:6, 10, 24). This prediction comes with a condition and a deadline. If he does not set the Israelites free, locusts would come into Egypt the next day. This emphasis on plagues taking place or ending "tomorrow" occurs elsewhere in this book (Exodus 8:10, 23, 29; 9:5, 18). The tenth plague (Exodus 12:29) will occur in the middle of the night when most people are sleeping.

Locusts are actually grasshoppers who experience certain weather or food patterns. When conditions are right, the grasshoppers produce a generation of offspring which look slightly different, act aggressively, breed quickly, form into swarms, and have an enormous appetite. These are "locusts," and they have caused problems for humanity for thousands of years. A large swarm of locusts can eat crops down to nothing in a matter of days, or even hours. The swarm predicted here will be colossal: like nothing ever seen in Egypt (Exodus 10:5).