Theory:

     EVERY afternoon, as they were coming from school, the children used to go and play in the Giant’s garden.
 
     It was a large lovely garden, with soft green grass. Here and there over the grass stood beautiful flowers like stars, and there were twelve peach-trees that in the springtime broke out into delicate blossoms of pink and pearl, and in the autumn bore rich fruit. The birds sat on the trees and sang so sweetly that the children used to stop their games in order to listen to them. “How happy we are here!” they cried to each other.

     One day the Giant came back. He had been to visit his friend, the Cornish ogre, and had stayed with him for seven years. When he arrived he saw the children playing in the garden.

     “What are you doing here?” he cried in a very gruff voice, and the children ran away.
Explanation:

It was a beautiful paradise-like garden where children used to play every afternoon once they return from school. Children loved that garden as there was no other better place for them to play near their location. But, unfortunately, it belonged to a Giant who was so selfish.
 
The garden looked so attractive for its extraordinary ambience. It was a beautiful, big garden filled with soft green grass and trees, and the pretty lawn grass had some beautiful star-like flowers on it. Twelve peach trees gave out delicate pink and pearl flowers during the spring and produced rich fruits during autumn. Cute little birds had occupied those wonderful peach trees and sang sweet songs which were pleasing to the children's ears. Those birds' songs were so melodious that they sometimes made children stop playing and listening to them. The children had happily spent their time inside the garden.
 
Children in the giant's garden.jpg
Children in the giant's garden
 
But, unfortunately, their happiness didn't last for a longer period. The so called-owner of the garden came back. As mentioned earlier, the garden owner was a selfish giant who lacked humanity and kindness.
 
For all those seven years, he stayed along with his friend Cornish ogre. 'Cornish ogre' is a Giant of a country named Cornwall in the U.K.
 
On returning from his friend's place, he was shocked to see children playing inside his garden as he was a selfish person. He couldn't tolerate them playing there. So he did not want to allow anyone to enter his garden. On being angry, he screamed at them, asking, "What are you doing here?".

The giant's voice scared all the children playing in the garden. So they ran away, thinking not to invite trouble for themselves.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Gruff Rude and unfriendly
2.
Scream
To make a loud shout as because you are hurt or scared
3.
AmbienceThe atmosphere of a place
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.