At its most basic sense, to "tree" is the action of spending prolonged periods of time in a singlespot. With this, the goal is often to adopt the lifestyle/lifecycle of a tree, which just consists of sitting in one spot its whole life and dying. Why, this term is quite subjective, as spots can range from someone living in the same city their whole life to being penned in one small room for days on end.
Man 1: "Come one, Jimmy, why won't you join us?"
Man 2: "I think Jimmy is highly depressed; he hasn't even left his house in weeks, let alone engaged in social interactions."