The Idea of "Ein Sof"
Kabbalah does not begin with the revealed Name or attributes of God given in Scripture, but rather with God's concealed essence. In that regard, Kabbalah's approach is similar to other forms of medieval "natural theology" that attempt to understand God's existence or attributes through the exercise of human reason alone. However, since human reason is limited in this venture, and we can only say what God is not rather than saying what God is (negative theology). Consequently, God is called Ein Sof (אין סוף), "the Infinite" or "Boundless" (the Hebrew word ein means "there is not" and refers to "no-thingness," and the word sof refers to an end or limit, so "Ein Sof" refers to "Infinite Nothingness," suggesting God's absolute transcendence from all forms of Being). In other words, God is beyond all our predications, descriptions, and apprehension. God's inner essence or nature is hidden from us and is therefore unknowable.
The idea that God's essence is essentially unknowable was fully articulated by Maimonides (1135-1204) in his work Guide for the Perplexed:
God's existence is absolute and it includes no composition and we comprehend only the fact that He exists, not His essence. Consequently it is a false assumption to hold that He has any positive attribute... still less has He accidents which could be described by an attribute (Guide 1:58).
Maimonides calls predicates ascribed to God's essential nature "negative attributes," since these amount to the negation of something we experience. Thus to say "God is omnipotent" means that God is not powerless; to say "God is omniscient" means that God is not ignorant, and so on. "Attributes of action," on the other hand, represent the self-revelation of God. These include the attributes of Creator, Redeemer, Source of life, etc. To say "God is Creator" is therefore known through the revelation of God's actions (namely, Scripture and creation itself). Most of the Names and Titles of God are therefore descriptions of God's attributes of action, though Maimonides reserves the special Name YHVH (יהוה) as the sole exception.
The Problem of Religious Language
Ultimately the problem of understanding God's essence concerns the limits of human language and meaning. If we say that God is infinite (i.e., non-finite) the words we normally use to describe finite creation suddenly become inadequate. (Even in mathematics we run into paradoxes regarding infinity: are there as many prime numbers as there are nonprime?) The attributes we use to describe finite beings cannot be directly applied to infinite Being without ambiguity. Indeed, some philosophers have said that the very idea of "infinity" is incomprehensible or incoherent. We see this in the case of the "predicate of existence," for example. God's existence is not like the existence of finite and contingent things that are subject to change, dissolution, and decay. Therefore we cannot say that "God exists" without qualification; we cannot ascribe properties to God based on the observed properties of creatures. This has led some theologians to say God's existence is "necessary," by which is meant that God cannot not exist, though this mode of existence is entirely different from all other forms of contingent existence known...
When it comes to language about God, either the predicates we use are equivocal (they mean entirely different things when applied to things in the world than they do for God), or they're univocal (they mean the exactly the same thing), or they're analogical (they mean more or less the same thing):
For Maimonides (and many Kabbalists), the "Torah is written in the language of men," by which is meant that there is a great gap between language about the world and the language about God. Saying "God is merciful," for example, means that actions of God are interpreted as merciful by human beings (analogical reasoning), but strictly speaking, there is no correspondence or "relation of similarity" between God's essential nature and human beings, and therefore we cannot say this is a true statement about God's essence. This distinction sometimes leads to paradoxes in theological discussion, when the attributes of action are confused with God's essential (or "negative") attributes.
Ultimately the idea of Ein Sof is an attempt to answer the Heideggerian question, "Why is there something, rather than nothing at all?" Ein Sof is an attempt to provide coherence to the idea of a "uni-verse" and to solve the ancient philosophical quandary of the "One and the Many" -- i.e., to find a unifying aspect behind everything. The problem of trying to account for this monistic expression of ultimate unity finds its way into problems regarding the use and limits of religious language.