This is the 6483rd most frequent Polish word.
cieśninie
"Cieśninie" is not a correct Polish word. Perhaps you meant "cieśnina," which translates to "strait" in English—a narrow passage of water connecting two larger bodies of water.
In this sentence, 'cieśnina' refers to a natural narrow waterway connecting two larger bodies of water.
Wąska cieśnina łączy dwa morza.
The narrow strait connects two seas.
Here, 'cieśnina' is used again to denote its geographical meaning, emphasizing the act of crossing it.
Powinniśmy przepłynąć przez cieśninę przed zachodem słońca.
We should cross the strait before sunset.
In this context, 'cieśniny' is the plural form, used to describe multiple straits visible on a map.