This is the 17099th most frequent Polish word.
wąską
"Wąską" means "narrow" (in the accusative or instrumental singular form of a feminine noun).
Here, 'wąską' is used as an adjective agreeing with the feminine singular noun 'ścieżką' (path) in the instrumental case, describing its characteristic as narrow.
Idąc wąską ścieżką, zobaczyliśmy piękne kwiaty.
Walking down the narrow path, we saw beautiful flowers.
In this sentence, 'wąską' is an adjective modifying the noun 'książkę' (book) in the accusative case, indicating its slender shape.
Wzięliśmy wąską książkę, bo była łatwa do przeniesienia.
We took a thin book because it was easy to carry.
Here, 'wąską' functions as a descriptor for the noun 'ulicę' (street) in the accusative case, implying the street's narrow aspect in its physical structure.