This is the 68th most frequent Croatian word.
jedna
"Jedna" means "one" (feminine form) in English.
Here, 'jednu' is the accusative form of 'jedna', used to describe 'knjigu', a feminine noun in the accusative case.
Vidjela sam jednu knjigu na stolu.
I saw a book on the table.
In this sentence, 'jedna' means 'one' and is used as a numeral to indicate the singularity of the bird, conforming to the feminine gender of the noun 'ptica'.
Jedna ptica leti visoko.
One bird is flying high.
In this instance, 'jedna' is used to specify one individual from a group, adhering to the feminine attribute of the implied subject.