This is the 8121st most frequent Russian word.
лев
Lion.
Here, 'лев' is used in its basic noun form to refer to the animal, the lion.
Лев — это царь зверей.
The lion is the king of beasts.
In this case, 'льва' is the genitive case of 'лев', used to describe the statue depicting a lion.
На площади стоит статуя льва.
There is a statue of a lion on the square.
Here, 'лев' is in the nominative case, indicating the subject of the action.