At the very bottom of the Tree of Life we find Malkuth, the Bride of Microprosopus, also called the Kingdom, the final phase of active manifestation. While Yesod is the machinery sorting and combining all emanations from the upper levels of the Tree, it is in Malkuth that these emanations finally find a solid base to materialize.
Malkuth is associated with Nefesh, the animal soul, and to Assiah, the world of making. It is in Malkuth were we are grounded and have our actual life, it is from Malkuth that we can look at the upper spheres at all. Being the very ground and body in which we are existent it is of utmost importance to our being, without Malkuth we simply would not be here at all. All spiritual exercise we will ever engage in has to be rooted and secured in our own Malkuth.
While Malkuth is securely anchored in the world of Assiah it combines all four elements in it, which becomes quite natural when we look at all life in and on our Earth. Take a little blade of grass for example.The grass itself is matter that is Earth, it was brought to life by the spark of vitality that is Fire, it is nurtured and fed by Water and it is Air in which it breathes. You could not take one of the elements away without destroying the being of this blade of grass, and it is likewise with all other life on Earth.
In the nice game of climbing the Tree of Life upwards and downwards in order to explore one's self, the desire is mostly focussed on the shining crown of Kether, believing that when the crown is reached the big goal is achieved. But, people forget that the biggest mystery is hidden in the own inside, and only very few have really seen the own Malkuth, let alone explored it.
See below the Tarot cards corresponding to Malkuth - the Tens and the Princesses: