The woody bookshelf that briefly appears in the Last Jedi teaser might just be a regular old space tree, but we doubt it. In early Episode VIII photos, a large gnarled tree sat right in the middle of the set. Many fans assumed that the prop was a "Force tree," which is actually a fairly big part of Star Wars' backstory.
In The Clone Wars' fifth season finale, a giant Force-sensitive tree grows in the middle of the Jedi Temple and watches over young Jedi as they train. In Marvel's Shattered Empire miniseries, Luke Skywalker learns that two small twigs from the tree survived the Emperor's reign. Luke gives one plant to Shara Bey, Poe Dameron's mother, and keeps the other for himself. It seems quite likely that Luke planted the sapling on Ach-To, the planet where he's training Rey in the trailer, and that it grew up during Luke's exile.
There's another, wilder option, too. In the Star Wars Rebels cartoon, an ancient creature named Bendu—who just so happens to have bark-like skin—helps the exiled Jedi Kanan Jarrus and his apprentice, Ezra Bridger, learn the ways of the Force. Interestingly, while Bendu is an extremely powerful Force user, he isn't loyal to either the light or the dark side. Instead, he says, he's "in the middle," and represents the Force's "center."
Given that Rey utters "balance" right as the wood appears, it's easy to see how some fans are connecting the giant tree to Bendu and not the Jedi Temple. Of course, there's the pesky fact that Bendu seemingly died (or at least disappeared) in Star Wars Rebels' third season finale, but death is hardly the end in Star Wars. Obi-Wan Kenobi's ghost shows up all the time, and Darth Maul found a way to survive after he was cut in half and dropped into a pit. In a galaxy far, far away, pretty much anything is possible.