IPA: /rɪˈrɪərˌwɔrd/
KK: /rɪˈrɪərˌwɔrd/
A group of soldiers positioned at the back of an army to protect it from attacks from behind.
The rereward ensured that the main forces could advance without fear of being ambushed.
Rereward is formed from "re-" (meaning again) and "reward" (from Old French *recompense*, meaning to give back or recompense). The word refers to giving a reward again or a second reward.
Think of receiving a reward 'again' ('re-') — that's why rereward means to give a reward a second time.