This is the 10449th most frequent Polish word.
czujna
"Alert" or "vigilant."
Here, 'czujna' is used as an adjective describing the subject 'ona', denoting attentiveness and vigilance.
Ona zawsze jest czujna podczas jazdy samochodem.
She is always vigilant while driving a car.
In this context, 'czujne' (a declined form of 'czujna') applies to an animal and conveys awareness to environmental sounds.
Zwierzę w lesie jest bardzo czujne na dźwięki.
An animal in the forest is very alert to sounds.
The word 'czujni' is adjusted for plural usage and emphasizes the importance of being perceptive and careful.