This is the 8187th most frequent Polish word.
olsza
"Alder" (a type of tree).
Here, 'olsza' refers to a type of tree called 'alder', used in a botanical context.
Olsza to drzewo często rosnące nad brzegami rzek.
Alder is a tree often growing along riverbanks.
In this sentence, 'olsze' is the plural form of 'olsza', used to describe multiple alder trees.
W ogrodzie zasadziliśmy trzy olsze.
In the garden, we planted three alders.
Here, 'olsz' is a declension of 'olsza', referring to alders in a possessed descriptive context, emphasizing location and abundance.