This is the 3rd most frequent Serbian word.
на
"На" in Serbian means "on" or "onto."
The word 'на' here is used to indicate a position or location, meaning 'on'.
Књига је на столу.
The book is on the table.
The word 'на' is used to indicate a direction or destination, meaning 'to'.
Идемо на плажу.
We are going to the beach.
The word 'на' is used to indicate the means or manner, equivalent to 'in' in English.