What is a Worry Tree, exactly?
It's a Cognitive Behavioural Therapy (CBT) technique for thinking through a worry in a structured way. You start with the thing on your mind, decide whether there's something you can actually do about it, and then either follow that thread to a small set of next steps or set the worry down with a brief technique for easing it.
The point of the structure is to interrupt rumination — the loop of thinking that doesn't go anywhere. anxietrees doesn't invent the technique; it gives you a calm place to do it.