This is the 8082nd most frequent Polish word.
nisko
"Low."
Here, 'nisko' is used as an adverb to describe the altitude at which the bird is flying.
Ptak lata bardzo nisko nad ziemią.
The bird flies very low above the ground.
In this sentence, 'nisko' modifies the adjective (through implication) to indicate a spatial positioning of the floor.
Moje mieszkanie jest na nisko położonym piętrze.
My apartment is on a low-lying floor.
Here, 'nisko' is used to describe the depth or pitch level of the chords played.