“Breaking of the vessels” (Shvirat ha-kelim) is a basic Kabbalistic concept describing what happened prior to the reality in which we live.
All of reality is composed of lights and vessels; and when the lights are too numerous for the vessels, the latter break because they cannot contain the light.
This breaking of the vessels creates an intermingled reality that lacks clear direction and meaning, for the vessels become entities discrete from the light.
Instead of revealing the light, they conceal it.
To illustrate this, when letters compose a word, they become a vessel containing meaning (or light), but when the word is “broken” and the letters separated, they no longer express that light.
–Rabbi Adin Steinsaltz