We might say that the Truth (note the caps) is an answer, and that this is a book that wants answers – whether it's the answer to a clue on $20,000 Pyramid, the identity of the person who is sending Miranda the strange notes, or the way time travel works. All of the characters in the book are searching for various kinds of Truths. As we learn, though, Truth is one of those concepts that can be hard to grasp. We will often see characters in When You Reach Me searching after insight, following clues, lifting veils, and untying knots in search of the hard-to-grasp Truth.
In When You Reach Me, learning the truth is a healing experience. For example, when Miranda learns the truth about why Sal has stopped spending time with her, she can begin to heal their friendship.
In the novel, untying knots is a symbol of puzzling out the truth.