This is the 4739th most frequent Polish word.
nocnych
"Of the nights" or "nighttime" (genitive plural form).
The word 'nocnych' is used as an adjective describing 'ulicach' (streets), indicating that these streets exist or are experienced during the night.
Uwielbiam spacerować po nocnych ulicach miasta.
I love walking through the nighttime streets of the city.
Here 'nocnych' modifies 'projekcji' (screenings) as an adjective, signifying the screenings that occur at night.
Kupiłem bilety na nocnych projekcji w kinie.
I bought tickets for the nighttime screenings at the cinema.
In this instance, 'nocnych' is an adjective for 'zwierząt' (animals), identifying the animals as being active or observable during the night.