The Tree of Life is a series of pathways connecting ten spheres (eleven if you count Daath) which explains the energetic movement through life. The aim of these articles is to give an oversight of the energy of the Archangel governing a particular sphere and how to connect with the Archangel.
The Tree is a very in depth study, far more so than I can cover and I am not an expert. If you wish to know more there is quite a lot of information on the net and many books on the Kabbalah or Quabbalah.