It is a counted stretch of time that links two major holidays and gives that stretch its own discipline.
The Omer is the count from Passover to Shavuot
My Jewish Learning explains that the Omer refers to the 49-day period between the second night of Passover and Shavuot. The name comes from the sheaf offering once brought in the Temple.
That historical reference matters because the count began in agricultural ritual and later took on additional religious meaning.
The counting links liberation to revelation
My Jewish Learning notes that the counting connects Passover to Shavuot, and in later Jewish tradition it also came to connect the Exodus to the giving of the Torah.
That is why the count matters so much. Freedom is not treated as the endpoint. It moves toward covenant and responsibility.
The period took on a semi-mourning tone
My Jewish Learning also explains that the Omer period is often marked by customs of restraint and mourning, which is why Lag BaOmer feels like a break in the middle.
Why it still matters
The Omer still matters because Judaism often uses counting to make time conscious. Instead of letting seven weeks vanish, the tradition makes them visible and cumulative.
The shortest accurate answer
The Omer is the forty-nine-day counting period between Passover and Shavuot, linking the Exodus to the giving of the Torah.