When the specific impulse is higher, less fuel is needed to make the rocket perform at a certain level. So, a fuel is more efficient if the specific impulse is higher.
w ɛ n WORD_BOUNDARY ð ə WORD_BOUNDARY s p ə s ɪ f ɪ k WORD_BOUNDARY ɪ m p ʌ l s WORD_BOUNDARY ɪ z WORD_BOUNDARY h aɪ ə ɹ WORD_BOUNDARY l ɛ s WORD_BOUNDARY f j uː ə l WORD_BOUNDARY ɪ z WORD_BOUNDARY n iː d ɪ d WORD_BOUNDARY t ə WORD_BOUNDARY m eɪ k WORD_BOUNDARY ð ə WORD_BOUNDARY ɹ ɑ k ɪ t WORD_BOUNDARY p ə ɹ f ɔ ɹ m WORD_BOUNDARY æ t ə WORD_BOUNDARY s ɜː t ə n WORD_BOUNDARY l ɛ v ə l WORD_BOUNDARY s oʊ WORD_BOUNDARY ʌ WORD_BOUNDARY f j uː ə l WORD_BOUNDARY ɪ z WORD_BOUNDARY m ɔ ɹ WORD_BOUNDARY ɪ f ɪ ʃ ə n t WORD_BOUNDARY ɪ f WORD_BOUNDARY ð ə WORD_BOUNDARY s p ə s ɪ f ɪ k WORD_BOUNDARY ɪ m p ʌ l s WORD_BOUNDARY ɪ z WORD_BOUNDARY h aɪ ə ɹ WORD_BOUNDARY