Think of the "tree" as a jigsaw puzzle. We've all done jigsaw puzzles as kids, right? Spreading out all the pieces on the kitchen table for weeks at a time. We know all the pieces will fit together eventually, but we just don't know how exactly, yet.
Often when working on a large jigsaw puzzle, we'll have several small clumps of connected pieces and several larger clumps of connected pieces, and usually one very large overall mass or framework of connected pieces.
That largest mass of connected pieces is the "big tree".