This idiom originally means "a bird startled by the mere twang of a bow-string". It is now used to describe a badly frightened person (due to past bad experiences).
In the Warring States period, there was an expert archer called "Geng Ying." His archery skills are unparalleled in his time. One day, he and The King of Wei were on the road and suddenly saw a group of birds flying above their head.
Geng Ying said to The King of Wei: "My king, can I shoot the birds flying in the sky?"
The King of Wei said: "I know that you are an expert in archery, you don't have to shoot them down."
Geng Ying spoke again: "I can shoot without using arrows, I just need to pull the bow to shoot the bird down."
The King of Wei was very suspicious and asked: "Can archery achieve such a skill?"
At that time, there was a lonely bird flying slowly. When the bird flew near, Geng Ying raised the bow and pulled the string. The bird flew closer and closer, and Geng Ying released the bowstring. As the "thump sound" slammed into the sky, the lonely bird that was flying suddenly fell.
The King of Wei was shocked. He didn't understand what was going on. Geng Ying gave The King of Wei an explanation: "The lonely bird was flying low and slow because it has been injured by an arrow. Because it flew slow, its companion was getting farther and farther away. Its wound had not healed yet, and the companions were leaving, the bird was very scared. When it suddenly heard the bowstring sound, his dread to the arrow made it lose its balance and fell from the air."
Later, people regarded this story as the idiom "a bird startled by the mere twang of a bow-string". This idiom means that people who have been frightened many times, when suddenly encountering the same terrible situation, will be frightened and uneasy, and do not know how to deal with it.
The photo is taken from here.