Theory:

When the moon rose he flew back to the Happy Prince. “Have you any commissions for Egypt?” he cried. “I am just starting.”

“Swallow, Swallow, little Swallow,” said the Prince, “will you stay with me one night longer?”

“I am waited for in Egypt,” answered the swallow.

“Swallow, Swallow, little Swallow,” said the Prince, “far away across the city I see a young man in a garret. He is leaning over a desk covered with papers, and in the glass by his side there is a bunch of withered violets. His hair is brown and crisp, and his lips are red as a pomegranate, and he has large and dreamy eyes. He is trying to finish a play for the Director of the Theatre, but he is too cold to write any more. There is no fire in the grate, and hunger has made him faint.”

“I will wait with you one night longer,” said the swallow, who really had a good heart. He asked if he should take another ruby to the young playwright.
Explanation:
 
When the night arrived, the moon rose in the sky. It was time for the swallow to leave the city. So he went to Happy Prince and asked him whether he had to do any work while going to Egypt. Also, he said that he was going to start his journey. At that time, Happy Prince requested swallow to stay with him for one more night. Later, the swallow replied that he needed to go to Egypt because his friends waited for him.
 
Then the Happy Prince repeated, “Swallow, Swallow, little Swallow”. He told the swallow about a man who was staying far away, across the city. The man was living in a small room at the top of a house. He was lying over a desk on which the papers were spread all over. And there was a glass nearby him in which the withered flowers were lying. Then Happy Prince described the young man to the swallow. He said that his hair was brown and crisp, which meant that he had not taken a bath for a long time. His lips were red like a pomegranate, and he had big dreamy eyes. It means that the man was very ambitious and hardworking.
 
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Papers spread all over the place

Furthermore, the Happy Prince said that the man was trying to write a play. And he had to give the play to the director after writing it. But he did not have the strength to write anymore because he was feeling very chill. The man had no money to put fire in his fireplace and also felt very hungry. Due to it, the man became very fragile.
 
Later, the swallow understood what Happy Prince was trying to tell him. He knew that Happy Prince wanted to help the young man. So he agreed to stay back for one more night. The swallow had a good heart, and so he thought of helping the prince. And further, the swallow asked Happy Prince whether he wanted to take another ruby and handover it to the young man.
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
Wither
Of a plant become dry and shrivelled
2
PlaywrightA person who writes plays or drama
Reference:
National Council of Educational Research and Training (2006). Moments. The Happy Prince – Oscar Wilde (pp. 28-35). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.