When Adam sinned in the garden of Eden, he was punished with the curse that: “thorns and thistles shall sprout for you…by the sweat of your brow you shall eat bread”.
The Midrash tells us that this verse gives us only part of a conversation that took place between Adam and Hashem; when he was told that his harvest would consist of thistles, Adam cried out “should I and my donkey eat from the same trough?”
Hashem replied that this would not have to be the case; Adam would not have to eat the same food as the animals. He would be able to eat bread – the food of humans – but only if he made the effort, and worked “by the sweat of his brow”.
The Omer derives its name from the quantity of barley that was brought as a sacrifice the beginning of the Omer period. At the end of the Omer, on Shavuot, another special sacrifice is brought – two loaves of bread. The very food – bread – that was so strictly forbidden on Pesach at the beginning of the Omer– is now transformed into something that is not only permitted, but is sanctified.
The Jewish people’s rôle in their own liberation at the time of Pesach was almost totally passive. Hashem brought the ten plagues; Moshe pleaded before Paroh, and the people merely did as they were told. On Shavuot, however, they took an active part. By saying "na'aseh v'nishma" - we will do and we will understand - they made the transition from reactive to proactive, from acting like donkeys, to acting like people.
For this reason they merited to bring an offering of bread – human food – on Shavuot, when they were only able to bring barley – animal food – on Pesach.
As we re-enact receiving the Torah each year, and bring ourselves to say "na'aseh v'nishma" we can also merit the reward that was promised to Adam. We also can take our place as active people, not just as passive animals. But this only comes as a result of effort – “by the sweat of your brow” - by accepting the מצוותupon ourselves, every year, and every day.