“Speak to B’nei Yisrael and say to them: When you enter the land that I am giving to you and you reap its harvest, you shall bring the first sheaf of your harvest to the Kohen.” LEVITICUS 23:10 (The Israel Bible™)
The omer is an offering of barley brought to the Beit Hamikdash in Yerushalayim on the second day of Pesach, corresponding to the sixteenth day of the month of Nissan. Only once this offering was brought, all grain that had taken root prior to the time of the offering may be eaten. According to the Sages (Kiddushin 38a), it was on the sixteenth of Nissan that the Israelites ran out of manna after it ceased to fall following the death of Moshe. The offering of the first grain in the Beit Hamikdash on that day each year reminds us of the eternal lesson of the manna. We dedicate a portion of our crops to our Creator before we eat from them ourselves, to remind us that no matter how hard we work the land, and despite the tremendous human effort required to produce it, our sustenance is really a gift from God in heaven.