Rashi contrasts the Arbeh, Locusts, in the time of Mitzrayim with the locusts that are described in Sefer Yoel. In Parshas Bo, the Plague of Locusts in Mitzrayim consisted of the most locusts of one single type gathered together in one swarm than any other time in history. When the locusts swarmed in Sefer Yoel, there were four different species. That was a record in its variety, but no one species was as numerous as in Mitzrayim.
The Ramban points out that the pasuk commands about the locusts in Sefer Yoel, עליה לבניכם ספרו – “Tell your children about it” (Yoel 1:3). The word ספרו is used in reference to that arbeh. In Parshas Bo, when Hashem instructs Moshe to go to Pharaoh, the Torah states, ולמען תספר באזני בנך; the same root word, ספר , tell, is utilized.
When the Torah uses the same word (or similar words containing the same root) in two places, it signals a connection, indicating that the two events noted are similar in nature. This is how we derive many halachos; this method of learning is called a gezeirah shavah. By utilizing the word תספר in Parshas Bo, and the word ספרו in Yoel, the Torah is showing that a similar topic is being discussed. Just as in Yoel the word ספרו was in reference to a swarm of locusts, so too the Makkah that Moshe was supposed to derive from the word תספר was an impending swarm of locusts.
