A small action that has big consequences.
Camels can carry large quantities. But there is a limit. Once you pass that limit, even a tiny amount will be too much for the camel. So a small piece of straw could break its back.
So the animal idiom expression means that a small action can have a big consequence as a cumulative action.
He was too demanding a boss. The straw that broke the camels back was when he asked me to work late on my birthday. I resigned on the spot.
I had a disastrous morning and the straw that broke the camel’s back was when my train was cancelled.
After a difficult few days, the straw that broke the camel’s back was when my car refused to start.