Like the tree in ESB and the pit in TLJ, they seem to be areas strong in the force, and both Luke and Rey encounter visions there, seemingly 'calling' to them, and perhaps part of the dark side. Luke of course the Vader with his head underneath; both a warning to Luke and a bit of a hint as to his parentage; while Rey seemed to see infinite reflections of herself...but no parents.

The old EU seemed to imply that the tree at least had some connection to perhaps a past battle that Yoda fought with a dark jedi, and that the tree's own aura helped to shield Yoda's own. However, this was mainly a thread dropped in later novels. I don't think even Zahn really followed up on it at all, especially since the prequels kind of messed up his own version of the Clone Wars.

The Disney EU did imply that there's force sensitive trees of some sort (Shattered Empire) and of course the Jedi library in TLJ is a big tree, although apart from that it doesn't seem to have any force stuff going on, at least compared to the pit.

It's been suggested that dark Rey is another tree vision. The background does seem slightly similar to the Dagobah tree as well, with what looks like almost a solid structure in the background along with all the mud and branches. Rey is also in a similar position to "Vader".