It means if someone can't post on their designated day, the understudy pinch-hits a fic to take their place. It's a very stressful job! Usually we have 1/2 last-minute drop-outs, so the understudy can save the day for the whole team. I believe Zel saved Team AU last time by posting a whole replacement fic on less than 24 hours' notice (a team is penalized if someone fails to post their fic on the day they're supposed to).