This is the 642nd most frequent Catalan word.
propi
"Propi" means "own" or "proper" in English.
Here, 'propi' denotes possession, emphasizing the book belongs to him specifically.
Ell va portar el seu propi llibre a la classe.
He brought his own book to class.
In this usage, 'pròpia' (feminine form of 'propi') describes confidence as a trait inherent or typical of great leaders.
Va actuar amb confiança pròpia de grans líders.
He acted with confidence characteristic of great leaders.
'Propi' in this sentence indicates something characteristic or typical of a certain group, in this case advanced students.