Symbolism, Imagery, Allegory
The Prayer Tree is a giant tree that stands in a clearing off the Jellicoe Road. In a way, it seems like an attempt to make graffiti cool and natural—Taylor notes, "it's the trunk that fascinates me most. There are carvings and symbols and messages and history" (9.26). Basically, over the years dozens of kids from Jellicoe have carved names, declarations of love and hate, and song lyrics into the trunk, which inspires Taylor to wonder about the people responsible for the stories they depict.
Things get personal between Taylor and the Prayer Tree, though, when she sees first the lyrics to a song the boy in the tree sings to her in her dreams, then the names of the five carved into the trunk. As result, Taylor says, "At night the Prayer Tree becomes my shrine" (10.11), the place she goes in an attempt to discover more about Hannah's characters (while she suspects they might have been real people, we aren't quite there yet).
In a way, the Prayer Tree symbolizes the connection between Taylor, her parents, and their friends, bringing the past and present together. It provides clues to Taylor about the identities and personalities of the five, including their musical tastes, as the magic of Google lets Taylor discover the songs that meant enough to them to immortalize them in the tree trunk.
You could also say that her discovery of Matthew 10:26, which Hannah carved shortly after Webb's disappearance, is Taylor receiving a message from Hannah herself that "Whatever is now covered up will be uncovered" (9.83). And indeed, the Prayer Tree helps Taylor do just this.