Board Thread:Questions and Answers/@comment-24731329-20150303101022/@comment-460323-20150303142239

Assuming that you meant the bar at the side of the map, it represents the 'HP' of the boss. Every time you hit/sink the boss, some of the bar is lost as 'damage' proportional to how much you deal to the boss. To complete a map with such bar, you'll have to deplete the bar first and then 'destroy' the bar by sinking the boss during the last chunk of the bar.

I'm not sure if sub drop rate is affected by HQ level, but I think they do.