This is the 2528th most frequent Polish word.
niska
"Low" or "short" (feminine form).
In this sentence, 'niska' is used as an adjective describing the shelf as being low in height.
Książka stoi na niskiej półce.
The book is on a low shelf.
Here, 'niska' describes the voice, indicating a low pitch.
Jej głos jest niska, co nadaje mu ciekawego brzmienia.
Her voice is low, which gives it an interesting tone.
In this context, 'niska' refers to the temperature, indicating it is low in comparison to usual values.