Theory:

“TILLOO! How often have you been told not to go that way?”

     “But why does Daddy go there every day?”

     “Because that’s his job, Tilloo!”

     'That way’ was an underground passage. Tilloo’s father went every day for work along that passage. And every day he came back from that direction. What lay at the end of that forbidden route? Not only Tilloo, but a great majority of the community would have liked to know. Tilloo’s father was one of the select few permitted to go that way. The above conversation invariably took place between Tilloo and his mother whenever he insisted on following his father.
Explanation:
 
The lesson "An Alien Hand" is written by Jayant Narlikar. He is an Indian astrophysicist and honorary professor at the Inter-University Centre for Astronomy and Astrophysics. The lesson is about a boy named Tilloo, and he is the protagonist of the lesson.

The lesson opens with a conversation going on between Tilloo and his mother. Tilloo's mother scolded him by stating how often he had been told not to travel through the underground passage. But Tilloo, on the other hand, was a naughty boy who desired to take the restricted route. While his mother was scolding Tilloo, he had a question in his mind. As a result, he approached his mother with a question, wondering why his father used to go there every day. The reason behind the statement was the boy wished to know why he was not permitted to use that way when his father could.
 
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Mother scolding Tilloo

When Tilloo inquired about his father's frequent visits to the forbidden way, his mother explained that it was his father's profession, and he had to go that route to get to work. Here "that way" refers to the "underground passage" where Tilloo's father used to go to work. Also, he would return from his work in the same direction.
 
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The underground passage where Tilloo's father used to go to work

On the other hand, there remained a common question about what was hidden under the forbidden route or the mystery behind the restricted path. Later, the narrator stated that not just Tilloo but the majority of the people were interested in knowing what was laid under the banned route. In addition, he said that it was not easy for everyone to use the underground passage. The reason behind his statement was that Tilloo's father was using it because he was one of the members of the special team who was permitted or received special authorization to use it as a part of his job.

Furthermore, the narrator stated that Tilloo's father was one of the members who had been chosen to use that underground passage. The statement implies that it was not safe for everyone to use the restricted way, and only a few selected members could use that passage.

Later the narrator said that the above conversation was constantly going on between Tilloo and his mother whenever Tilloo insisted on following his father. From the above statements, one can easily understand it was not safe to use the underground passage.
  
Meanings of the difficult words:
 
S.No
Words
Meanings
1
Underground Below the surface of the earth; below ground
2
PassageA usually long and narrow part of a building with rooms on one or both sides, or an enclosed path that connects places
3
DirectionThe position towards which someone or something moves or faces
4
Permitted To allow something
5
Community The people living in one particular area or people who are considered as a unit because of their common interests, social group, or nationality
6
Forbid Not allowed, especially by law
Reference:
National Council of Educational Research and Training (2007). Supplementary. An Alien Hand - Jayant Narlikar (66-73). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.