This is the 4597th most frequent Turkish word.
belgelerini
"Belgelerini" means "their documents" in English.
The word 'belgelerini' here is the accusative plural form of 'belge' with a third person possessive suffix, meaning 'his/her documents'.
Belgelerini masaya bıraktı.
He/she put his/her documents on the table.
In this sentence, 'belgelerini' refers to the documents belonging to the addressed person, using the possessive and accusative forms together.
Belgelerini tamamlamalısın.
You must complete your documents.
Here, 'belgelerini' indicates possession ('his/her') and also serves as the direct object of the verb 'hazırladı' (prepared).