We are all familiar with the mitzvah (Sefer Hachinuch, mitzvas aseh 306) of counting the omer. To the Rambam this mitzvah is still considered a biblical obligation while according to the Rosh and Shulchan Aruch and others, because we do not yet have the korban omer the counting nowadays is a rabbinic mitzvah, zecher l'mikdash - a reminder as to what we had, and please G-d will have.
What is an Omer? The first time this term is used in the Torah is (Shemos 16:16) when the mon descended in the desert and Moshe conveyed the Divine instruction that each individual was to gather an omer per person each day, the omer being an amount that satiated each person. The last verse in that chapter informs us that an omer is a tenth of an ephah which is the volume of approximately forty two and a half eggs (it is a dry measure.)
There is a theme in the Torah that repeats itself many times, namely, the consecration of firsts. The first born son is to be redeemed after thirty days. The produce that grows in the land of Israel cannot be eaten until the land owner designates the terumah (i.e. first) portion for the kohein and to the Levi, his tenth known as ma'aser. Thus, the annual bringing of an omer measure of barley in the Temple on the second day of Pesach as an expression of appreciation to Hashem for His giving us the barley crop is most understandable, and fits into the established pattern. What is rather perplexing is that this offering is referred to as the korban omer, as if the specific amount of barley brought played a significant role in this offering. Moreover, the Torah uses the term omer three additional times in Parshas Emor teaching 1) that the omer of barley is to be waved on the second day of Pesach 2) that in addition to the waving of the Omer a lamb is to be offered 3) that the mitzvah of counting, bridging the two holidays of Pesach and Shavuos, is to begin on the day you bring the Omer of waving. It seems most strange that the Torah should place such emphasis on what appears at first glance to be a very minor detail, i.e. how much barley (an omer's worth) constituted the offering.
Rav Yosef Salant zt''l, in his work B'aer Yosef, provides a most exciting response to this question. He says the Torah is calling attention to the mon by referring to the korban of thanksgiving for the barley crop as a korban omer. How so? He notes, as the Talmud (Taanis 9a) teaches, that the mun descended in merit of Moshe, and thus it ceased on the seventh of Adar in the fortieth year which was the day of Moshe's passing. The last portion of mon which the Jews received nourished them for the thirty day period of mourning for Moshe, for three additional days as they prepared to cross the Yardain river, and through the fifteenth Nissan, i.e. the first day of Pesach. On the second day of Pesach they began to eat from the produce of the land of Israel (see Yehoshua 5:11). It is thus understandable that to mark this transition from relying on mon to eating the produce of the land of Israel we bring an omer's worth of barley each year on the second day of Pesach in appreciation for the many omer of mon that had nourished us throughout our stay in the desert until the second of Pesach the year we entered Israel.
Moreover, since it was the mon taught us to have total reliance and faith in Hashem, that He is the provider of their sustenance, now that we would be planting and harvesting and deriving our food from the ground, and not from the sky, we could easily come to believe, "my strength and the might of my hand made me all this wealth" (Devarim 8:17.) Thus, the Torah said on that this day of transition bring an omer of barley to remember our daily sustenance of an omer of mon, for ultimately there is no difference between lechem min hashomayim and lechem min ha-aratz (bread made in heaven or that which sprouts forth from the earth.)
Rav Salant quotes the Medrash (Vayikra Rabba 28:3) in which Rabbi Berachya taught that Hashem directed Moshe to tell Bnei Yisrael that while Hashem in His generosity provided an omer per person in the desert, when we reciprocate and bring an omer to Him, it is but one omer on behalf of the entire nation, not an omer per person.
I believe if one takes the time to calculate the discrepancy between Hashem's generosity to us and our reciprocation, the numbers are astounding. There were approximately two million Jews in the desert (the Torah informs us there were six hundred thousand men between twenty and fifty, roughly an equal number of women, and when one adds in the young and older members of the community there must have been roughly two million Jews.) Thus, each week Hashem "special delivered" fourteen million omer of mon to the Jews; monthly, they received fifty six million; annually six hundred and seventy two million; over forty years they received approximately twenty six billion, eight hundred and eighty million portions of Omer of mon. And all He asks in return is one Omer of barley! The message is so powerfully penetrating: He gives us so much, and asks so little in return.