There was a painter in ancient China, and he painted dragons very well. One day, he drew four dragons on the wall. People saw the dragons and thought they were beautiful, but the dragons had no eyes.
The painter said “If you paint eyes, the dragons will fly away.” People didn’t believe him so he drew the dragon’s eyes and the dragons really flew away.
The meaning of this idiom is that adding something in an important place when writing an essay will make the essay better.