on fire=to be doing something really well

منظور از این اصطلاح این است که کسی عملکرد بسیار خوبی دارد/یا احساسات شدیدی را تجربه می کند

Our pitcher was on fire in the last inning. He struck out three batters in a row

Did you hear the guitarist in the last song? He was on fire! Fantastic playing