What is significant about the knothole and what does its "closure" represent?

Furthermore, the "closure" represented a disconnect with
Boo. If you watch how the story progresses, once they are no longer able to exchange
these items with Boo, his existence doesn't really come up for chapters. In fact, the
tone and atmosphere of the novel shifts completely from the children's very immature
pursuit of this innocent man to the adults' very real pursuit of a different innocent
man. Without that knothole being closed up by Mr. Nathan Radley, we as a reading
audience might have continued to hear from Scout from her very self-centered account on
their continued Boo Radley curiosities. As it is, the closure of this chapter to their
lives meant their curiosity had to take life somewhere else. This somewhere else
happened to be the courtroom in Tom Robinson's case.

I
think this closure symbolized a closure to the reality of childhood which includes a
realm of imagination and a belief in the good of people.