First, a moment of background, we need to understand that we are made up of many levels. Just as the universe bears within it many levels of complexity, man was created as a microcosm of the world, with all of its levels of complexity. And just as in the universe, each level effects all the other levels below and above it, so to in each person, each level is affected by, and in turn affects all the other levels of that person.
We know how many different levels there are that make up the body, how many interdependent systems that were one to fail, the body would cease to function. We bless HaShem each morning and many times throughout the day in acknowledgement of the wonder that is the body. Yet we know that just as the skin is the outermost layer of the body, the body is the outermost layer of the person.
The manifold layers of the soul are broken into five major levels, listed here in ascending order: Nefesh, Ruah, Neshamah, Chayah, Yechidah. Any of the first three can generally be translated as 'soul' though each of the five bears a very different nature. Note that we have a more basic 'animal' soul that animates the body, meaning that none of these five levels of soul is in any way directly observable in this world. The Nefesh, or lowest of the five levels of the soul is so refined that there is no perceivable similarity between the nature of the Nefesh and that of the body. (or even the animal soul) The nefesh is likewise coarse and unrefined in comparison to the Ruah, just as the body is to the Nefesh. This relationship is true for each of the various levels of the soul, until the level of Yechidah is so removed that its relationship to a person is tenuous a best.
When we are created/born, our awareness encompasses only the most tangible realities, the immediate report of the senses. As we grow and develop, we learn to refine our awareness to abstract levels beyond the raw information of our sensations. Reading for example requires us to recognize with our eyes the fine print of letters, to divvy up the words and sentences and to process how the words would sound, what they mean, and project into our minds the ideas of the one who wrote the letters. Reading is a very advanced abstraction, a very subtle understanding of the coarse feedback of the physical world.
In the very same way, we are initially only aware of the most basic and coarse spiritual realities. Only through growth and development can we work to understand and make sense out of what are spiritual senses report. This is a major aspect of the system HaShem created, both the physical and the spiritual. When we start to learn something new, we are immediately rewarded. In the physical world this is through natural opiates in the brain that reward new brain growth. In the spiritual realm this involves an initial glimpse of HaShem's Holy light. Once we have tasted the reward, then we can more easily recognize a particular form of stimulus. (be it physcial or spiritual) The initial sweet taste essentially tunes our awareness to know that there is something out there worth seeking. Every stage of growth works like this.
As we progress and develop spiritually, we are given new tools, new vessels to encourage and enable our further development. Each of these new tools is a new level of soul. There are a very great number of levels within each of the five levels we mentioned. Each new accomplishment is another rung on this ladder of spiritual development. Each new level of soul is a step inward, deeper towards the essence of what we are, a figurative fragment of HaShem.
Now we've returned to hopefully a place of understanding where we may try and take another step and gain a new insight. As we said previously, the Zohar speaks of the soul as our inheritance, a place in which to dwell in the world to come. If we think about it, it makes sense, in a way our soul dwells within in our body. Our bodies are the land for our soul. Similarly each level of our soul plays the part of an inheritance to the level of soul above it. As we refine ourselves further, we move deeper. We enter into new undiscovered lands, or heichalot-halls in the palace of the king, each one closer to the throne room than the last.
When we were perfecting a particular level, like learning to read in the physical world for example, we couldn't enjoy the benefit of this new level, this new tool. Instead we worked hard to gain some basic competence, some grasp of this level. Once we've grasped this level, and we have become proficient in it, then we can enjoy all the potential bound up in this new tool. Reading, again as the example, all manner of books and stories, exploring worlds of knowledge previously hidden behind the patience of others to expound them to us. With each level comes profound freedom and benefit.
Once we have mastered the particular level at which we exist, we begin to sense the depths that lie beyond the medium, the 'art form' of wielding the tool that comes only from a greater wisdom, a deeper understanding. In this very same way, we can see how each new spiritual ascension allows the previous level to transition into the home, the background, the proving ground for the next level.
This is how the Tzaddikim travel from level to level, world to world, always striving for the deeper next further level. In this way even one's soul becomes the inheritance, the land, the homestead of one's desire. The desire to push on, closer and ever closer to HaShem's oneness, until the soul is merely a means to an end, the end being the fulfillment of this sole (no pun intended) desire.
For the rest of us, when we hear "The Promised Land" we think of a place, a plot of land bordering on the plots of land of our extended families, where we bodily live, eat, and sleep in the comfort of HaShem's protection. But, for the Tzaddikim, their soul is The Promised Land where their desire can take comfort, can be fulfilled in HaShem's revelation.