what is the meaning of "Dropping like flies" I need both Eng

剑桥词典(Cambridge Dictionary):
[1] If people are dropping like flies, they are dying or falling down in large numbers: (像苍蝇落下)
死了很多人；跌了很多人...
[a] The heat was overwhelming and people were dropping like flies.
[2] to stop doing an activity in large numbers:
[a] There used to be over 20 of us in our aerobics class but they're dropping like flies.(像苍蝇落下)很多人停止做活动...