This is the 209th most frequent Polish word.
jedna
"Jedna" in Polish means "one" (feminine form).
The word "jedna" is used here as an adjective in its feminine singular form, signifying the number 'one' to describe the noun "książka" (book), which is feminine.
Jedna książka leży na stole.
One book is lying on the table.
The word "jedna" is employed here symbolically, emphasizing 'one' to denote a singular aspect ("strona," feminine) of the subject under discussion.
To jest tylko jedna strona medalu.
This is just one side of the medal.
In this sentence, "jedną" marks the feminine singular accusative case of the adjective, indicating a selected item ("sukienkę," feminine accusative singular) among multiple choices.