What does Flonase nasal spray 15ml Sig 2 sprays to each nostril qd-bid prn sinusitis mean?

This is what a physician would write on a prescription for the drug Flonase. It says that 15mL volume of Flonase should be sprayed twice to each nostril daily (qd) or twice daily (bid) as needed (prn) to treat sinusitis. Flonase is an nasally inhaled steroid that behaves essentially as a topically applied drug to the sinuses. It is used to decrease inflammation in the nasal passages.