The Omer refers to the 49-day period between the second night of Passover (Pesach) (April 6) and the holiday of Shavuot (May 25).
This period marks the beginning of the barley harvest when, in ancient times, Jews would bring the first sheaves to the Temple as a means of thanking God for the harvest. The word Omer literally means “sheaf” and refers to these early offerings.