This is the 12958th most frequent Russian word.
литр
Liter (unit of volume).
Here, 'литр' is used as a quantity measurement indicating the volume of the purchased milk.
Я купил литр молока в магазине.
I bought a liter of milk at the store.
In this sentence, 'литр' specifies an amount of water needed, demonstrating its function as a measurement unit.
Для приготовления супа нужно литр воды.
For making the soup, you need a liter of water.
The word 'литр' describes the volume capacity of the container, again used as a unit for liquid measurement.