The Self-Concealment - Helem Hatzmi
The Self-Concealment - Helem Hatzmi is the "reality" of the world as it is included in its source and root before it was created.
The Essence of Essence
[In examining] any reality, we cannot grasp its essential nature but only the way in which that essence clothed itself in a form and definition that can be perceived by the senses or intellect. For example, we recognize "wisdom" when we see a person with inventive talent, and "understanding" when we see a person with special explanatory ability. But the essence of wisdom as it exists detached from any specific idea (or the essence of the power of understanding before it clothes itself in a particular explanation) we cannot see, and they remain hidden even from the person themselves. Yet despite our inability to discern the essential nature of any thing, it is clear to us that everything has some hidden source - which is the essence of the thing.
Since all powers of the soul come from the essence of the soul, we must say that the essence of the revealed powers is already included in the encompassing powers of the soul, and similarly with Divinity. That is, everything in the world has a source in Divinity, and this source is the essence of that reality (and it is the reality of the thing as it is detached from all definitions and forms related to revelation). For example, from the fact that the body of fire is hot and luminous, we must say that the essence of fire (the sphere of fire beneath the sphere of the moon) is neither hot nor luminous.
The Self-Concealment
As it is with every particular created reality, so it is with the world as a whole. That is, in many places in Chassidus it is stated that the general nature of the world is "concealment" – that it conceals the Divinity that creates and sustains it. The Holy One, blessed be He, "in His goodness renews daily, perpetually, the work of creation," and if the letters of the Ten Utterances were to withdraw and return to their source and root, this entire world would revert to absolute nothingness as before the six days of creation. But the world conceals this from us. Thus, the definition of the world is a power of concealment.
That is, "Self-Concealment" is the concealment of the world itself (as if within G-d) even before it was created. It's as if the world already exists - but exists as a "non-existent existence." On one hand, it's impossible to "point to" the world within G-d - because it doesn't exist there. But on the other hand, it does exist there - except its existence is a "non-existent existence," the existence of something that doesn't exist. This is a special style and expression through which Chassidus describes the reality of the world as it exists, so to speak, within G-d even before He creates it.
The Desire of the Holy One, Blessed Be He
On the verse "His legs are pillars of marble," our Sages taught, "His legs - this is the world." This "non-existent existence" is, as it were, G-d's desire. Before the creation of the world, the "desire" (so to speak) of the Holy One, blessed be He, was hidden. In this hidden thing called the "legs" of the Holy One, blessed be He, there is a longing to be revealed, and this longing to be revealed encompasses all reasons for creation.
That is, the secret of creation is G-d's desire to create it, and this desire is to create His 'Self-Concealment' - to reveal that which is, as it were, hidden between Him and Himself. For additional "essential" matters, see here.