This is the 8585th most frequent Russian word.
озеро
Lake.
Here, 'озера' (genitive case of 'озеро') describes possession or relation, specifically the shore belonging to the lake.
На берегу озера растут красивые деревья.
Beautiful trees grow on the lakeshore.
In this instance, 'озеро' (nominative case) functions as the subject of the sentence, describing the lake directly.
Озеро было очень спокойным в этот вечер.
The lake was very calm this evening.
In this sentence, 'озера' (genitive case of 'озеро') indicates the location near which the action of walking will occur.