In the process by which God creates and sustains the entire system of the worlds, God restricts His infinite light through a graduated series of limitations and concealments that are known as tzimtzumim ("contractions").
The result is a graduated series of worlds, or levels of reality and perception, in which each lower world is characterized by a greater concealment of Godliness.
The term for this process is derived from the word shalshelet ("chain"), suggesting that the worlds are connected like the links of a chain.
The interconnection of the worlds is comparable, in the individual, to the interconnection between the realms of intellect, emotion, thought, speech, and action.
–Rabbi Adin Steinsaltz
In Opening the Tanya by Rabbi Adin Steinsaltz