This is the 70th most frequent Romanian word.
său
"His," "her," "its," or "their" (possessive pronoun).
In this sentence, 'său' is used as a possessive adjective referring to 'telefonul' (the phone), and it modifies the noun to indicate ownership by the subject 'El' (he).
El și-a pierdut telefonul său.
He lost his phone.
'Său' in this context is synonymous with 'its' in English, showing possession of the cover by 'caietul' (the notebook).
Caietul este pe masă, dar coperta sa este ruptă.
The notebook is on the table, but its cover is torn.
Here, 'său' is used in general way to denote possession of 'scopul' (the purpose) by an implied subject, reflective of the general 'everyone' group.