At the end of chapter three, Atticus reads to the children from the newspaper. One article is about a man who sat up on the top of a flagpole for no reason, so Jem gets it into his head to do the same. Jem spends his whole Saturday up in his treehouse just like the man in the newspaper article. Jem has Scout run errands for him as he is in the treehouse from sunrise to sunset so he doesn't starve or get bored. This is somewhat akin to what Boo Radley does as well. He stays in the house all day long, and as the rumors go, he goes out at night. Boo has had someone in his life to run errands for him, just like Scout does for Jem. The difference is that Jem tries this one Saturday, but Boo Radley does it his whole adult life.
Whether Jem stays in the treehouse to mimic the man on the flagpole, to get an idea of what it is like to be Boo Radley, or both, a symbolic connection can be drawn between the treehouse and the Radley house. The scene can also be considered a foreshadowing because it is Jem who later tells Scout that the reason Boo Radley doesn't come out of his house is simply because he doesn't want to.
The tree house represents peace and childhood innocence. It is hidden in the leaves away from all the evils of the world and allows the children to feel sheltered and safe in a "perfect" little world they create for themselves. Nothing is ugly or revealed and naked/exposed there.
In a way, the tree house is an extension of other mockingbird symbols, but isn't as prevelant and obvious as Boo or Tom and the relationships between people in the novel. It is simply a temporary safe haven into which the children may escape for a brief hiatus.