This is the 7th most frequent Greek word.
στο
The Greek word "στο" means "at the" or "to the" (used as a contraction of "σε" + "το").
In this sentence, "στο" is a contraction of "σε το" which serves as a prepositional phrase indicating location, specifying 'to the park'.
Πήγα στο πάρκο για περίπατο.
I went to the park for a walk.
Here, "στο" indicates the final position or state where the book is placed, expressing 'onto the shelf'.
Έβαλε το βιβλίο στο ράφι.
He/she put the book on the shelf.
Here, "στο" is again a contraction of "σε το" indicating a static location, meaning 'in the city center'.