The four worlds in Kabalah are a model for describing 4 stages of development of creation, as it is emerging, here-and-now, from the absolutely literally infinite source. A few notes:
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1) It is described as "the" source, singular, because, if it is absolutely literally infinite, there can be only one and no others. Lacking this, the entire construct collapses.
2) A beginner needs to know, these 4 stages or "worlds" ( I prefer to call them realms ) are not literal places. They are not physical. Not material. They are not limited by time-and-space. They are concurrent and omnipresent. They have no beginnings; they have no endings. That's what makes them "magical" for lack of a better word. This presents a significant challenge when describing them, because each and every word that can be chosen to describe them will be describing a "thing" which is fixed in time-and-space. But these are not "things" which are fixed in time-and-space. Any manner of categorizing them, placing them in order, or including them in a sequence will always and forever be at best a gross approximation of what is actually being described.
So, please try to keep in mind, if there is a process being described, "a process of creation, reality emerging, something from nothing", it cannot possibly be occurring in a manner which is consistent with a sequence of events. All of it is happening simultaneously. Here-and-now.
There aren't literally 4 worlds. There's nearly infinite worlds. One way for understanding the emerging of reality is to break it out into 4 stages. It's a model, a very useful mental construct. Nothing more, and nothing less.
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The 4 worlds in the Kabalistic framework are: Atzilus
אֲצִילוּת, B'riyah
בְּרִיאָה, Yetzirah
יְצִירָה, Assiyah
עֲשִׂיָּה. ( This is the sephardic transliteration ). These 4 worlds / realms, are 4 sections of a nearly infinite nested chain. One side of this chain is with/in the infinite source. The other side of the chain is reality as we know it with all of its particular details, plentiful diversity, this and that, big and little, before and after, life and death, joy and misery, and everything in between. There's a wikipedia entry on it. It's OK, but I think it over complicates the concept. Because these are immaterial concepts, concurrent and omnipresent, it's easiest to understand them as they exist in the human heart-and-mind. Doing so, this is how I describe the four worlds. Once that's understood, it's easier to go back to the more traditionally definitions and descriptions understanding them as intended.
Atzilus means "emanating". In the creative process, this is the "eureka!", "ah-hah!", moment when the mind-and-heart collectively, simultaneously, realize that it's having an idea. It's the "gasp!" before the idea has landed in the psyche. It's described as "the heart skipping a beat". That's the realm of Atzlius.
B'riyah means "creating". Here the mind is having the idea lacking any form. It's like a cloud, or an endless sea, or a lexicon of concepts in a pool which is conglomerating. It's like the water above the water wheel, before there's a water wheel. It's pressure. Inertia. A swelling of will.
Yetzirah means "forming". The unformed conglomeration of ideas is taking shape, gathering, like a garment which is being tailored and stitched and woven up by many little fingers, quick like lightning.
Assiyah means "making" / "doing". The creation is completed and is serving it's function. The vessel is fully formed and is being filled. Vase, with flowers. A wheel on a cart, turning. The cork on a bottle, sealing. Etc. But not only purely material "things". Any and all objects, events, and ideas are emanating and manifesting, simultaneously, in this way, in Kabalah.
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The analogy that's often used, an example, is building a house. The important relationship, to focus on, are the "bookends", the extremes, Atzilus <<>> Assiyah. If one pays close attention to their own cognition while they, themself are having an idea, they'll notice these two are occurring simultaneously in their own psyche. Using the example of one who is building a house, imagine an individual wandering through the woods. The come to a clearing, and they have an idea to build a house there. When they first encounter the clearing, and the idea is emerging, the house, in general, and all of it's particular details, are simultaneously there-and-not-there, something-from-nothing, in the heart-and-mind. The one who is wandering through the woods, encounters the clearing, and has an idea, "something belongs here", and simultaneously, the idea of the house flashes into their awareness, they are having an epiphany. " ... a house would be perfect, right here, right now. " The idea from the divine mind is simultaneously emerging and manifesting so completely, that each is being enveloped and veiled with/in each other, something from nothing from something from nothing from something ad infinitum.
