This is the 5282nd most frequent Russian word.
озере
"Озере" is the prepositional case form of "озеро," meaning "in the lake."
In this sentence, 'озере' is in the prepositional case, describing the location where the action occurs.
Мы рыбачили на озере целый день.
We went fishing on the lake all day.
Here, 'озере' represents the location of the fish, also in the prepositional case.
Рыба в этом озере очень вкусная.
The fish in this lake is very tasty.
In this example, 'озере' indicates the location where lilies grow, in the prepositional case.