Theory:

     One morning the Giant was lying awake in bed when he heard some lovely music. It sounded so sweet to his ears that he thought it must be the King’s musicians passing by. It was really only a little linnet singing outside his window, but it was so long since he had heard a bird singing in his garden that it seemed to him to be the most beautiful music in the world. Then the Hail stopped dancing over his head, and the North Wind ceased roaring, and a delicious perfume came to him through the open casement. “I believe the Spring has come at last,” said the Giant; and he jumped out of bed and looked out.
Explanation:
 
As mentioned earlier, winter took over the giant's garden completely. The selfish giant didn't understand the reason behind the winter's endurance in his garden. It was his selfishness that resulted in such a terrible result. Days passed. One day, he woke up from his sleep listening to some sweet, lovely music that he never heard before. He remained in his bed and was listening to that melodious music, thinking it was made by the king’s musicians who passed by that road.
 
Actually, it was the music made by a linnet bird. It sang in the giant's garden happily for some time, which the giant thought to be the music of the king's musicians. Since spring had not entered his garden for many days, he forgot the voice of the birds. For him, the song of the bird seemed to be the sweetest music in the world. The endurance of winter in the giant's garden has come to an end.
 
Everything, including the North Wind, Hail, Snow and Frost, came to an end. The flowers began to blossom, and a pleasant perfume reached him through the window. The giant guessed that the winter had come to an end. He then happily jumped from his bed to observe the garden from his window. He was happy to see the winter's end in his garden after many months. He exclaimed in a delightful voice as "I believe the Spring has come at last".
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
CeaseTo stop something
2.
CasementA window that opens like a door
3.
LinnetA small bird with a short beak
4.
RoarTo make a loud, deep sound
Reference:
National Council of Educational Research and Training (2008). The Selfish Giant – Oscar Wilde (pp. 17-24). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.