Literal Meaning: digging a hole in the wall and stealing the light
Metaphorical Meaning: It is used to describe that someone studies very hard.

In the West Han Dynasty (202 BC to 9 AD), a son named Kuang Heng, was born in a peasant family. When he was young, he was eager to go to school. However, his parents were so poor that they could not afford his school expense. When he was a little older, a relative of his taught him how to read and write and then he possessed the ability of reading books.

But Kuang Heng was too poor to buy any books. He had to borrow books from the others. At that time the books were very expensive. The book-owners usually would not like to lend their books to others. Kuang Heng had to do short-time work for the rich people in the sowing and harvest seasons. What he earned was not money, but the opportunity of reading books.

Several years passed and Kuang Heng grew old enough to be the main labor force of his family. He was busy in his farmland all day and had a little time to read books at noon. It usually cost him ten or fifteen days to finish one book. Kuang Heng was very anxious about this. He thought he had to spend some time reading in the evening. But he had no money to buy the light oil. What could he do?

One night, Kuang Heng lied on the bed and recited books he read at day. Suddenly he saw a beam of light going through the wall in the east side. He stood up and went there to have a look. It was the light which was generated by the oil light of his neighborhood and went through the seam of the wall. Seeing this Kuang Heng had a good idea. He took a knife and dug a hole in the wall so that there was more light through the wall. He could read books just with this light.

Kuang Heng studied so hard and became a scholar years later.