This is the 1321st most frequent Polish word.
nieba
"Nieba" means "heaven" or "sky" in the genitive form in Polish.
Here, 'nieba' is in the genitive case used as a part of the preposition 'na tle nieba', which means 'against the sky'.
Patrzę na ptaki szybujące na tle nieba.
I am looking at the birds gliding against the sky.
In this sentence, 'nieba' is in the genitive case, used to indicate origin, as in 'from the sky'.
Nie mam złotego orła z nieba.
I do not have a golden eagle from the sky.
Here, 'nieba' is in the genitive case as part of the phrase 'piękno nieba', meaning 'the beauty of the sky'.