This is the 11187th most frequent Turkish word.
dönün
"Turn" (imperative form, plural or formal).
Here, 'dönünce' is a derivative of 'dönün', used to indicate a condition or time by showing the action of turning or returning.
Dönünce ne demek istediğimi anlayacaksınız.
You will understand what I meant once you return.
Here, 'dönün' is used as a command in the second person plural or polite form equivalent to 'please turn' or 'turn around'.
Dönün ve yeni yolu gör.
Turn around and see the new path.
In this sentence, 'dönün' again acts as a polite or plural imperative verb, instructing someone to take a turn.