to preserve and nurture one's spirit (idiom); honing one's strength for the big push
Definitions (Wiktionary)
phrase, idiomatic
to recuperate and build up energy
紅軍雖弱，卻養精蓄鋭，以逸待勞。
红军虽弱，却养精蓄锐，以逸待劳。
hóng jūn suī ruò ， què yǎng jīng xù ruì ， yǐ yì dài láo ． But the Red Army, though weak, has conserved its strength and stored up its energy, and is waiting at its ease [for the fatigued enemy].