It's the international travel document issued by your country of nationality. Italian passports are issued by the Questura. Foreign passports stay valid in Italy as long as they're still recognised and not expired.
Your passport is the document that proves your nationality and lets you travel internationally. Italian authorities expect non-EU residents to keep theirs valid at all times.
"After her passport expired in Iesi, Yara had to book an appointment at her embassy in Rome to renew it. Until then, she avoided international travel and kept the receipt of her application with her permesso."
Your permesso doesn't replace your passport. Even with a valid permit, you'll need a valid passport to fly out of Italy and to enter most other countries.
Official source
Polizia di Stato
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