Rashi
the first of His grain Like the first of the harvest before the Omer, which it is forbidden to eat, and whoever eats it is liable, so will all those who eat him be guilty. So did Jonathan render it.
Tze'enah Ure'enah
“Israel was holy to the Lord, the first fruits of his harvest” [2:3]. Israel is compared to holiness. Just like terumah, which is holy and an Israelite should not eat it, but only the priests. If an Israelite eats the terumah, he is strongly punished. In the same way, God will punish whoever persecutes Israel. They are the first fruits, that grow first and no person should eat of them before they bring the Omer offering.12Rashi, Jeremiah, 2:3.