Theory:

     SOAPY moved restlessly on his seat in Madison Square. There are certain signs to show that winter is coming. Birds begin to fly south. People want new warm coats. And Soapy moves restlessly on his seat in the park. When you see these signs, you know that winter is near.

     A dead leaf fell at Soapy’s feet. That was a special sign for him that winter was coming. It was time for all who lived in Madison Square to prepare.

     Soapy’s mind now realised that fact. The time had come. He had to find some way to take care of himself during the cold weather. And, therefore, he moved restlessly on his seat.
Explanation:
 
"The Cop and the Anthem" is a short story written by O. Henry. The story is about Soapy, a homeless man, and he is the protagonist of the story.

The first scene takes place at Madison Square. Soapy was sitting in the park and seemed to be moving restlessly on his seat. There were a few signs in the surroundings representing that the winter season was arriving. They were: the migration of birds from one location to another in search of food and shelter; people's need for new coats and sweaters to keep them warm. And also, Soapy moved here and there restlessly on his seat. While seeing all these indications, one can understand the winter season was coming nearer.
 
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Migration of birds from one place to another

When Soapy was sitting on the bench, he noticed a dried leaf falling at his feet. It was a unique sign that indicates the seasonal change. When he spotted the dried leaf, he assumed it was sending a message about the winter season. So Soapy thought it was time for all the people in Madison Square to face the upcoming winter. They had to be ready with the necessary items, such as woollen clothes, sweaters, boots, and blankets, to keep them warm.
 
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Soapy was sitting on a bench in the park

Being a homeless man, Soapy had stayed in the park for several nights. But Soapy's mind understood the fact that the time had come for him to leave the park and find a better place to stay. He knew that he wouldn't be able to manage the chillness produced by the cool climate if he remained in the park during the winter season. So Soapy had to find a new place to take care of himself during the winter. This was the reason behind Soapy's restlessness, so he moved here and there on his seat.
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
FactA thing that is known or proved to be true
2
Homeless A person without a home, and therefore typically living on the street or park
3
RestlessUnable to rest or relax as a result of anxiety or fear
4
SignAn object or event whose presence or occurrence indicates something is going to happen
5
Winter The coldest season of the year, in the northern hemisphere from December to February and in the southern hemisphere from June to August
Reference:
National Council of Educational Research and Training (2007). An Alien Hand. The Cop and the Anthem- O. Henry(20-29). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.