This is the 2748th most frequent Turkish word.
üstüne
"Üstüne" means "on it," "onto," or "over it" in English.
Here, 'üstüne' is used to indicate the spatial position, specifying the surface of an object (the table) where something (the book) is placed.
Kitabı masanın üstüne koydum.
I placed the book on top of the table.
In this usage, 'üstüne' conveys a metaphorical sense, suggesting focused attention or consideration upon a subject.
Bu durumun üstüne düşünmelisin.
You should reflect on this situation.
In this sentence, 'üstüne' specifies the location where an additional ingredient (chocolate) is applied, again indicating a spatial relationship.