This is the 75th most frequent Greek word.
σου
"Σου" means "your" or "yours" in English.
In this sentence, "σου" denotes possession, indicating that the book belongs to the person being spoken to.
Το βιβλίο σου είναι στο τραπέζι.
Your book is on the table.
Here, "σου" is used as an indirect object pronoun, showing to whom the action of telling is directed.
Σου είπα να έρθεις νωρίτερα.
I told you to come earlier.
In this context, "σου" shows possession again, specifying that the story being referenced belongs to the person being addressed.